Since the sixth century, the Bulgars had known intermittent contact with the Christians of the surrounding nations, whether as merchants or prisoners-of-war or through diplomatic relations. During the later eighth and early ninth century, the Christian population in Bulgar lands increased so much that Christians were rumored to have influence at the court of Khan Krum (802-814); they were also persecuted under Khan Omortag (814-31). The Bulgars continued to remain “officially” pagan until the reign of Khan Boris, who came to power around 852.
Several factors may have led Khan Boris to assume a more favorable attitude towards Christianity. First, Christianity offered a belief-system that transcended — at least potentially — cultural or ethnic boundaries and thereby offered a means not only to unify Bulgaria’s disparate populations but also to secure legitimacy and respect with Byzantium and the West. The ideology of Christian rulership also enhanced the position of the prince vis-à-vis his subjects including the often contentious boyars. Furthermore, Boris’ sister had converted to Christianity while a hostage in Constantinople and may have influenced her brother. Finally, Boris himself seems to have been attracted to Christian beliefs and practices, as evidenced by the seriousness with which he pursued the conversion of his people.
Boris’ move towards Christianity seems to have begun in earnest with the opening of negotiations in 862 between himself and Louis the German for an alliance against Ratislav of Moravia. News of the alliance reached the Byzantines and they attacked Bulgaria preemptively, taking advantage of the weakness caused by famine in that year. Boris surrendered in 864 and by mid-865 had probably been baptized. It was around this time that Patriarch Photius (858-67; 977-86) sent Boris a letter in which he instructed Boris on the basic tenets of orthodoxy and exhorted him to adhere to the principles of Christian rulership. Greek missionaries were sent to Bulgaria to speed the process of conversion but within a year, Boris sought to distance himself from the patriarch in Constantinople and sent a legation to Rome to open negotiations with Pope Nicholas I (858-67) about Bulgaria’s movement into the Roman sphere of influence. Letter 99, sent back to Bulgaria with Bishops Formosus of Porto and Paul of Populonia as well as a collection of books and liturgical equipment, records the pope’s response to the Bulgarians’ questions and problems. Indeed, because of the format of Nicholas’ responses, this letter seems to offer a relatively undistorted look at the problems that the Bulgarians themselves thought christianization posed to their culture and the specific aspects of their new faith about which they were curious. It is therefore a priceless document for the study of the process of christianization in the early Middle Ages.
Pope Nicholas I: Responses to the Questions of the Bulgars A.D. 866 (Letter 99)
You wish to know if anyone is permitted to perform any labor on Saturday or Sunday. Concerning this matter the oft-remembered holy Pope Gregory said, while addressing the Romans: It has come to my attention that certain men of a perverse spirit have sowed some depraved things among you which are contrary to the holy faith, so that they forbid anything to be done on Saturday. What else should I call such people except preachers of the Antichrist, who shall, when he comes, make Saturday and Sunday be kept free from any work? For because he pretends that he died and rose again, he wishes that Sunday be held in veneration, and, because he compels the people to judaize in order that he may recall the exterior rite of the law and subject the perfidy of the Jews to himself. Indeed, as long as what is said through the prophet: “Do not carry burdens through your doors on Saturday”[Jer.17:24] can be maintained, so long was the law allowed to be observed according to the letter. But after the grace of omnipotent God, our Lord Jesus Christ, appeared, the commandments of the law, which were spoken through figures, cannot be kept according to the letter. For if someone says this commandment concerning the Sabbath should be preserved, he may say that it is necessary the sacrifices of flesh also be performed, he may also say that the commandment concerning the circumcision of the body should also be retained. But let him hear the apostle Paul saying against him: “If you are circumcised, Christ does you no good.”[Gal.5:2] Thus, we understand and maintain spiritually what has been said about the Sabbath. For Sabbath means “rest”. But we consider the Redeemer himself, our Lord Jesus Christ, to be the true Sabbath. Also, the person who recognizes the light of his faith, introduces burdens through his gates on Saturday, if he drags sins of concupiscence into his mind through his eyes. Therefore we do not introduce burdens through our gates on Saturday, if we, established in the grace of our Redeemer, do not drag the weights of sin into our soul through our corporeal senses. For our very Lord and Redeemer is read to have done many things on Saturday, so that he reprimanded the Jews, saying: “Which of you does not release his ox or ass on the day of the Sabbath and lead it to water?”[Lk.13:15] Therefore, if Truth itself ordered through itself that the Sabbath not be kept according to the letter, whoever keeps the leisure of the Sabbath according to the letter of the law, who else does he contradict than Truth itself? But on Sundays one should cease from earthly labor and devote oneself to prayers in every way, in order that whatever act of negligence has been committed during the other six days, may be expiated with prayers throughout the day of the Lord’s resurrection.
 Gregory I, Register XIII.1.
You ask whether you should cease from earthly work on the feast days of these apostles, martyrs, confessors, and virgins. Yes, [you should cease from work] on the feasts of the blessed virgin Mary, of the twelve apostles, of the evangelists and of their precursor, the lord John, of St. Stephen the Protomartyr as well as on the birthdays of those saints whose celebrated memory and feast day shall be held among you by God’s favor. It should be clearly known, that one should cease from worldly work on feast days in order that the Christian may be able to go more easily to church, to engage in psalms, hymns, and spiritual songs,[cf. Eph. 5:19; Col. 3:16] to spend time in prayer, to offer oblations, to communicate with the memories of the saints, to rise to imitate them, to concentrate on divine scriptures, and to distribute alms to the needy. If someone, neglecting all of these things, wishes to spend his time only in leisure and turns his licit labors to other vanities of the world, it would be better for him to work with his hands on that day, obeying the command of the blessed apostle Paul, so that he might have something to give to those suffering want, just as the person who, although he says that the Lord ordained that he who proclaims the Gospel, should live by the Gospel[cf. I Cor. 9:14], nevertheless does not use this power, but works with his hands lest he burden one of those to whom he preaches the Gospel. [cf. I Cor. 4:12; I Thess. 2:9; II Thess. 3:8]
Because you ask whether it is permitted to carry out judgment on the feasts of the saints and whether the person, if he deserves it, should be sentenced to death on this same day, you should know that on those feasts on which, as we have shown, one should cease from all worldly labor, we think that one should abstain all the more from secular offices and especially from killing. For although both can perhaps be exercised without fault, nevertheless it is fitting that since a person should cling more tightly to the things which are of God, he completely cut from himself the things which are of the world, especially since a person who comes to divine military service (militia) should not be implicated in secular business. [cf. 2 Tim. 2:4] Furthermore, because of the reverence of so great a festival, it is appropriate that nothing be introduced, unless it is something which bring forth joy, peace and happiness for all. But with this said, the law will teach you sufficiently on which days besides these no secular judgments should be exercised.
Among your questions and inquiries, you claimed to request secular laws. Regarding this matter, we would willingly have sent the codices which we thought might be necessary for you at present, if we knew that one of you was able to interpret them for the rest; if we have given some books concerning secular law to our messengers, we do not want them to be left [with you] when they return, lest by chance someone interpret them for you in a perverse way or violate them with some falsity.
 It is worth noting the close association that the Bulgars seem to assume between becoming Christian and gaining “secular laws,” a phrase that Nicholas I understands as law codes written in books (codices), probably containing Roman law. Cf. King Æthelberht of Kent’s production of the first known Anglo-Saxon law code in the wake of St. Augustine of Canterbury’s mission to the Anglo-Saxons in 596, which is briefly described by Bede, Historia Ecclesiastica II.5. The model in this case seems to have been the Lex Salica.
You also indicated that a certain lying Greek claimed that he was a priest, although he was not, and baptized many in your country. Then, when by God’s inspiration you recognized that he was not a priest, you condemned him to lose his nose and ears and to be killed with the harshest of beatings and then to be cast out of your country. Behold there was fulfilled in you — forgive us — what the Apostle proclaimed about some persons, namely that they had the zeal of God but not according to knowledge.[cf. Rom. 10:2] To be sure, the fellow acted badly, if he lied, but you showed a zeal in this action which, while pious, was not well considered. For although this fellow pretended that he was a priest, his simulation nevertheless conferred salvation on a great many. Truly, there have been diverse people who announced Christ in different ways, yet they did not do so for the sake of Christ;[cf. Phil. 1:15-18] instead, they proclaimed him either for some other purpose or any way they liked. Yet the Apostle, who strove as much as possible solely in order that Christ might be proclaimed, did not prohibit these men, because Christ had indeed ordained that he who proclaims the Gospel, should live by the Gospel. [cf. I Cor. 9:14] Therefore, if many people were preaching Christ to acquire earthly comforts, Christ was not at issue, but rather avarice; again, because many preachers of the Gospel were distinguished by the faithful with the great honors, some men therefore preached Christ in order that they, too, might receive the like; but this does not mean that Christ was in their intention, but rather envy. Yet not one of these persons was forbidden so that Christ would not be proclaimed through them; even Judas, who was sent by the Lord among the other apostles, did miracles and proclaimed Christ, and baptized many in His name. And so, as Anastasius, bishop of the apostolic see, writes: One does not ask, who or what kind of person preaches, but rather Whom he preaches.
 Letter to the Emperor Anastasius, ed. A. Thiel, Epistolae Romanorum pontificum genuinae et quae ad eos scriptae sunt, Hildesheim/New York, 19742, I: 622.
You also asked whether the people who received their baptism from this man are Christians or should be rebaptized. Now then, if they were baptized in the name of the highest and individuated Trinity, they are clearly Christians, and it is not fitting that they be baptized again by any Christian, because, as the aforementioned apostolic pope Anastasius wrote to his Emperor of the same name, … and the baptism, which may be far from a church and have been given by an adulterer or a thief, comes unimpaired to the one receiving the gift: for that voice, which sounded out through the dove, excludes every spot of human pollution and declares and says: “Here is the one who baptizes.”[Jn.1:32] If baptism comes unimpaired to the person receiving the gift, even though it was given by an adulterer or a thief, why does it not therefore stand unimpaired and without need of repetition when some worldly fellow, pretending for I do not know what reason to be a priest, has offered it? Certainly Acacius, former bishop of Constantinople, pretended to be a priest, after he was condemned by Pope Felix. But let us hear what the aforementioned Bishop Acacius said about him; among other things, he said: When he – namely Acacius – claimed the name of priest for himself, although he had been condemned, the tumor of pride was inflicted upon his own head, because it was not the people, who thirsted after his gift in the mysteries, that was excluded, but rather it was only the soul which had sinned, that was properly liable for the just judgment, as numerous passages of scriptural instruction attest, and this is true because, as that most famous apostle says, “neither the person who plants, i.e. catechizes, nor the one who waters, i.e. baptizes, is anything; rather it is God who gives the increase.[I Cor. 3:7] Hence, when evil men administer good things, they pile damage not upon others, but upon themselves, and therefore it is certain that no portion of injury shall touch those whom that Greek baptized, because It is He who baptizes, i.e. Christ, and again: God gives the increase; with “and not man” left understood.
 This passage is from the same letter of Pope Anastasius as in n.9.
You who thus condemned this person may wonder whether you should do penance for it. Clearly, every sin is washed away with penance, which profits only when God’s grace accompanies it. For the Lord looked first at Peter and then Peter broke down in tears. Now then, it is obvious that you have committed a sin against that man. First, because, as far as we are able to understand the matter, it was not yet clear that the man was not a priest, and it is written: Do not judge before the time.[I Cor. 4:5] Second, because although he pretended that he was what he was not, he nevertheless did nothing that is not pleasing to God and his faithful, especially since in this nation, where until that time no right faith, no right religion was practiced, he saw a door divinely opened for him to gain many [for God] and perhaps eagerly desired to lead many people to so great a gift, a desire in which he felt that God mercifully was helping him. Indeed, if David pretended to be raving mad, just so that he could achieve his own safety, [cd. I Kings 21: 13-15] what harm did this man do who saved so great a multitude of people in such evident need from the power of the devil and eternal perdition? In particular, although he pretended to be a priest, in baptizing people he nevertheless did not do that which only a priest is fit to do in a case of such evident necessity; we therefore believe that he pretended to be a priest, because he felt that you had already been persuaded that the mystery of baptism was only allowed to priests. Third, [you have committed a sin against this man] because, although in this simulation he was truly culpable, in the conversion of so many men, he was worthy of much praise. Fourth, because, although he deserved punishment, the punishment nevertheless should not have exceeded the measure of vengeance; nor should [the punishment] have heaped upon one person so many and such cruel injuries, since, after his nose and ears had been cut off, expulsion from your country would have been a sufficient punishment for him, instead of the amputation of his members which he experienced at your judgment.
Now then, you have told us about how you received the Christian religion by divine clemency and made your entire people be baptized, and how these people, after they had been baptized, rose up unanimously and fiercely against you, claiming that you had not given them a good law and also wishing to kill you and establish another king; and how you, having been readied against them with the help of divine power, conquered them from the greatest to the least and held them captives in your hands, and how all the leaders and magnates along with every one of their children were slaughtered by the sword, though the mediocre and lesser persons suffered no evil. Now you desire to know whether you have contracted any sin on account of those who were deprived of their lives. Clearly what you did not escape without sin nor could have happened without your fault, was that a child who was not privy to their parents’ plot nor is proven to have born arms against you, was slaughtered along with the guilty, although innocent. For after the Psalmist said: I shall not go to my seat in the counsel of vanity and with people who do iniquitous deeds, I have hated the gatherings of the wicked and I shall not sit with the impious, [Ps. 25:4-5] he says a little while later in this regard, while praying to the Lord: Do not destroy my soul with the impious nor my life with the men of blood.[Ps. 25:9] Furthermore, the Lord declares through the prophet Ezechiel, saying: Just as the soul of the father is mine, so, too, the soul of the son: only the soul that has sinned shall perish;[Ez. 18:4] and a little later he speaks about the father: But he bore a son, who, when he saw all the sins which his father had done, was afraid and did not do anything like them, he did not eat upon the mountain nor lift his eyes to the idols of the house of Israel, he did not violated the wife of his neighbor or trouble the husband, he did not keep surety nor commit robbery, he gave his own bread to the hungry and covered the naked with clothing, turned his hand from the injury of the poor man, did not accept usury and any superabundance, judged my judgments, and walked in my commandments: this man shall not die in the iniquity of his father, but shall live with life. His father, because he falsely accused and did violence against his brother and did evil in the midst of his people, behold he died in his own iniquity. And you say: Why does the son not bear the iniquity of the father? Because his son did judgment and justice, kept all my commandments and carried them out, he shall live in life. Only the soul that has sinned shall die: the son shall not bear the iniquity of the father and the father shall not bear the iniquity of the son.[Ez. 18:14-20] You also should have acted with greater mildness concerning the parents who were captured, that is, [you should have] spared their lives for the love of the God Who delivered them into your hands. For thus you might be able to say to God without hesitation in the Lord’s prayer: Forgive us our debts, as we forgive our debtors.[Mt. 6:12] But you also could have saved those who died while fighting, but you did not permit them to live nor did you wish to save them, and in this you clearly did not act on good advice; for it is written: There shall be judgment without mercy for the person, who does not exercise mercy;[James 2:13] and through the abovementioned prophet the Lord says: Is it my will that the wicked man should die, sayeth the Lord God, and not that he be converted from his ways and may live?[Ez. 18:23] But because you erred more because of your zeal for the Christian religion and your ignorance than because of any other vice, with subsequent penance seek mercy and indulgence for these sins through the grace of Christ.
You indicate that you wish to know what you should do concerning those who reject the Christian law. In this matter it should be noted that every man, just as he has contracted sin through the fault of another, that is, through Adam, from which he needs purgation through the water of baptism, so, when he comes to profess his faith at the wave of rebirth, he chooses another as a certain kind of father, namely a spiritual one, by whom, once he has been received, he is instructed, protected with complete protection, and presented again to the rector of the church as if he was some kind of surety. Furthermore, if he has rejected what he promised, he should by all means be recalled by the man who received him and was the mediator of his faith. But if he will not listen to the man whom he himself had offered as surety for himself,[cf. Mt. 18:12] he should be reported to the Church, by which, acting with great zeal, it is fitting that this sheep be led back to its proper flock [cf. Jn. 14:6] and recalled in every way, so that it may return to the way of truth, which is Christ. Moreover, the Church should persuade him like a mother, like a teacher, so that he may see that he has imitated an apostate spirit, about which it is written that it did not stand in truth.[cf. Jn. 8:44] Also consider what Peter, prince of apostles, proclaims, when he says: But if, after they have escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ, they are again entangled therein and overcome, the latter end is worse with them than the beginning. For it would have been better for them not to have known the way of righteousness than to have turned from the holy commandment delivered unto them, after they came to know them. But it has happened unto them according to the proverb: `A dog is turned to his own vomit again,’ and `The sow that was washed, wallows in the mire’. [I Peter 2:20-2] Finally, if he does not heed the Church, let all truy consider him to be a heathen, i.e. a pagan, and because of this, he may now be rightly oppressed by the external powers as an outsider. It is also certain that God often arouses the powers against the deniers of Christ, against the desertors of baptism. Therefore let no one be surprised that God incites the powers, so that that these men may bend their necks and return, humbled, to the Church. For after Hagar had left her mistress, an angel came to her and said to her: “What is this, Hagar servant of Sarah? Return to your mistress.”[Gen. 16:8-9] Therefore, whoever is like this, is afflicted for this reason — that he may return. And would that he does return because he shall receive the promised inheritance along with his brothers! Indeed, unless the Christian power were aroused against people of this kind, how would they render an account of their rule to God? Certainly it pertains to the Christian kings of the world to want to keep their mother Church, from which they were spiritually born, peaceful and undiminished during their reigns. Even King Nebuchadnezzar, who had erected a statue of himself and had forced all the people to adore it, is read to have said, after being moved by the praises of the three boys who were freed from the fire: And I proclaim a decree among all the tribes and all the tongues in all the land and whoever shall speak any blasphemy against the God of Sidrac, Misac, and Abdenago, shall be destroyed and their houses laid waste.[cf. Dan. 3:29] Hence, if a barbarian king raged so, lest the God of Israel be blasphemed because He could free the three boys from temporal fire, how much more should Christian kings rage, because Christ is denied and mocked, Who liberated not merely three boys but the whole world along with these kings from unending fires! For indeed those who are convicted of lying or being unfaithful to God and Christ, are either never or hardly ever allowed to live by Christian kings: and how could they not be indignant and become enraged, when they see Christ, who is the king of kings and lord of lords,[I Tim. 6:15] denied and mocked by men and the fidelity, which was promised to Him, not maintained? Let kings act more zealously with the zeal of God,[cf. I Macchabees 2:54] because they rememberl that they have been more liberated from hell by Christ more than did the king who attended to the three boys snatched from the flames.
What rightly should be done about those who have risen up to kill the king, the venerable laws which we sent to you in writing shall adequately teach you. This matter nevertheless remains within the judgment of the king, who should forgive the sinner not merely once, but seven times seventy times,[cf. Mt. 18:22] in accordance with what the Lord commanded of Peter in the holy Gospel.[cf. Lk. 7:47] For the person whom one forgives more, loves more, and, as the Gospel teaches: The king would have forgiven his servant a debt of ten thousand talents, i.e. many monstrous sins, if the servant himself had wished to forgive his fellow servant a debt of one hundred denarii, i.e. minor sins.[cf. Mt. 18: 24-33]
What should be adjudged concerning a free man, who shall have slipped from his country in flight, if he is apprehended? Clearly, nothing should be adjudged, except that which the laws decree. Nevertheless, the sacred histories claim that many of the saints departed from their homeland and were venerated more highly in another. Indeed, unless I am mistaken, I have found that, as long as there are no other reasons, they were never culpable for their departure alone, especially since it is said to our father Abraham: Depart from your country and your kin,[Gen. 12:1] and some of our forefathers lived for many, many years in a land that was not their own. Moreover, if someone does not dare leave his country, he is not free; and if he is free and rightly is not held to be bound by the bonds of any condition, he is not fleeing but rather leaving his country, just as it is not said to Abraham: “Flee”, but rather “Depart from your country”; and because he did this out of obedience, he went forth, with no one assigning any punishment.[cf. Gen. 12:4]
 For each of the following, Perels, the editor, cites a number of possible laws to which Nicholas might have been referring, including the Lombard laws, Carolingian capitularies, and Roman law, especially Justinian’s Code and the Digest. Since we do not know which legal codices he had in mind, and hence which specific penalties he felt permissible, it seems less misleading simply to refrain from such speculation and instead recognize that, in general, the penalties prescribed by law would have been much harsher to the point of capital punishment.
Translated by W. L. North from the edition of Ernest Perels, in MGH Epistolae VI, Berlin, 1925, pp.568-600.
Note: there is more chapters in this letter.
Photius I, Patriarch of Constantinople. The Patriarch and the Prince. The Letter of Patriarch Photios of Constantinople to Khan Boris of Bulgaria, English trans. D.S. White & J. Berrigan. The Archbishop Iakovos Library of Ecclesiastical and Historical Sources 6. Brookline, MA 1982. A translation of Photius’ letter to the Khan.
F. Dvornik. Les Slaves, Byzance et Rome au IXe siècle. Paris 1926.
S. Runciman. A History of the First Bulgarian Empire. London 1930.
M. Spinka. “A Study in the Spread of Byzantine Culture among the Slavs,” Studies in Church History 1 (1933): 25-36.
R. Sullivan. “Khan Boris and the Conversion of Bulgaria: A Case Study of the Impact of Christianity on a Barbarian Society,” Studies in Medieval and Renaissance History 3 (1966): 55-139. Reprinted as Essay IV in Christian Missionary Activity in the Early Middle Ages. Variorum Collected Studies Series CS431. Aldershot 1994. A most comprehensive survey of the sources, problems, and historical context along with a detailed assessment of Nicholas I’s Letter 99.