Byzantium retained the institute of slavery despite being a Christian state. Christian ideology was to come into a confrontation with social reality, but in fact both coexisted peacefully in many spheres. This paper aims to demonstrate the ways along which the integration of the social phenomenon of slavery with the religious institute of monasticism took place, i.e. how this process is reflected in laic and ecclesiastic law, as well as canonistics. Did the humanisation and mitigation of law occur over time? Was there any difference between the approaches of laic and ecclesiastical law, and canonistics? The article considers the question of slaves’ integration into monkhood in the context of the entire Byzantine millennium. The research is based on documents of civil law (the law of Valentinian III, the constitutions included in the Codex Justinianus, Justinian’s Novels, the Ecloga, Basilika, and Leo VI’s Novels), ecclesiastical law (the Canons of the Councils of Gangra and Chalcedon, and the apostolic rules), secular and ecclesiastical law (the Nomokanon in 14 titles), works of canonists (Zonaras, Aristenos, and Balsamon, Matthew Blastares), as well as some extracts from the Rules of Basil the Great, Pachomius, and the Life of Hypatius of Rufiniana. The article concludes that in Canon law and writings of canonists, the position of the Church is abundantly clear: a slave may not become a monk, cleric or bishop without his master’s authorisation. The canon of 451 was adopted once and for all, it did not undergo any change. It is under the terms of Justinian’s Novels that the humanisation of law can be stated. This Emperor granted all men, both slaves and free ones, the right (almost in equal measure) to follow their religious vocation. Having put on a monastic habit, a slave became conditionally free, for he was to return to his previous status when leaving his monastery.