Conversions from Islam to Christianity occurred quite often in the Frankish states of Outremer. Some Muslims converted and entered the service of Frankish rulers, such as Godfrey of Bouillon and King Baldwin I of Jerusalem; Muslim converts are also mentioned as serving in Frankish armies at various times. The conversion of a Muslim peasant is referred to in a Muslim treatise, Karāmāt Mashā’ikh al-Ard al-Muqaddasa (The Cited Tales of the Wondrous Doings of the Shaykhs of the Holy Land), by Diyā‘ al-Dīn al-Maqdisī. A papal letter of 1264 deals with poor Muslims (and Jews) who come to Acre (mod. ‘Akko, Israel) to be converted and enjoins the Latin patriarch to provide for their sustenance during the days on which they are to receive instruction. Some Franks married baptized Muslim women: a decretal of Pope Celestine III sent in 1193 to the bishop of Acre deals with Muslim converts wishing to marry Frankish women. A decretal of Pope Innocent III, sent in 1201 to the titular bishop of Tiberias (mod. Teverya, Israel), deals with recently baptized Muslims who, before conversion, married spouses related to them in degrees prohibited by canon law, and lays down that such marriages are valid. The ruling was reiterated in 1274. A Muslim convert called Mestre Jaques Sarasin le ypoticaires, noveau crestien (Master James the apothecary, a new Christian) collaborated toward the end of the thirteenth century in the writing of an Arabic-French glossary.
Evidence for the conversion of Muslim slaves is more abundant. Frankish custom granted freedom to a converted slave, and so Frankish lords reacted by preventing the conversion of their slaves so as to avoid the obligation to manumit them; in 1237-1238 Pope Gregory IX promulgated a compromise ruling that allowed slaves to accept baptism but abolished the custom that gave them freedom. As the lords’ resistance continued, the papal legate Odo of Châteauroux threatened them in 1253 with excommunication. Yet some baptized slaves did gain freedom. Thus, the will of a burgess drawn up in Acre in 1264 reveals that he owned two baptized slaves, at least one of whom he had manumitted, and that he ordered the baptism and manumission of two others. Treaties between the Mamlūks and Frankish rulers in the years 1267-1283 deal with fugitive slaves from the sultanate who converted to Christianity upon their arrival in Frankish territory: two treaties allow the fugitives to stay put, the third stipulates the return even of those who sought sanctuary in a church. The Muslim chroniclers Ibn al-Furāt and al- Maqrīzī relate that in 1268-1269 four mamlūks (slave soldiers) of Sultan Baybars I fled to Acre and converted there. Al-Maqrīzī mentions the abduction and forcible baptism of a Muslim girl and adds that the Franks frequently coerced Muslims to become Christian.
As for Christian attempts at missionizing, a Frankish hermit tried in the 1120s to preach Christianity to a Muslim ruler; in 1217 James of Vitry, bishop of Acre, preached in the Chris- tian-Muslim borderland, sent letters in Arabic to Muslims outside Frankish territory, and baptized a number of Muslims. In later years Franciscans and Dominicans systematically endeavored to bring about Muslim conversion, and Pope Gregory IX proclaimed their efforts as commendable as crusading. When a Dominican friar boasted in 1273 that he had baptized more than a thousand Saracens, his contention must not have been utterly divorced from reality.
There were also many instances of Franks converting to Islam. Both Latin and Arabic chroniclers relate repeatedly that Christian warriors who could not endure the hardships of battle crossed the lines and became Muslims. Such cases took place during the Second Crusade (1147-1149), in the times of Saladin, and during the Fifth Crusade (1217-1221). Also, many Franks who fell into Muslim captivity chose to embrace Islam and regain freedom. Conversions unrelated to warfare also took place. A law attributed to King Baldwin II of Jerusalem deals with a home lige (liege man) who abandons his fief, denies Christianity, and becomes a Saracen; another law lays down that children may disown parents who go to the land of the Saracens and become Jews or Saracens. A decretal of Pope Alexander III reveals that an archbishop of Tyre (mod. Soûr, Lebanon) wrote to him that, in his province, it frequently happened that a Christian went over to Saracen territory and renounced his faith. Pope Celestine III, in a letter of 1193 to the bishop of Acre, mentions a Christian who left his faith and wife and according to gentile rites married a pagan (that is, Muslim) woman who bore him several sons; after the death of the Christian wife, he decided to revert to Christianity and marry the pagan one, who had converted in the meantime together with her children. This case is paralleled by the story given by the Arab writer Usāma ibn Munqidh about a Frank who fell into Muslim captivity, became a devout Muslim, married the daughter of a pious family, but later fled to Frankish territory and reverted to Christianity together with his sons. A still more dramatic case of Frankish conversion to Islam appears in A Thousand and One Nights: a Muslim merchant who comes to Acre and lusts for the beautiful wife of a Frankish knight finds her after the battle of Hattin in a crowd of Frankish prisoners; she converts to Islam of her own will, and the two are married by the qadi (judge) of Saladin’s army [The Book of the Thousand Nights and a Night, trans. Richard F. Burton, 12 vols. (London: H. S. Nichols, 1897), 7:99-104].
There is also some evidence of Jews converting to Christianity and Christians converting to Judaism in the kingdom of Jerusalem.