Modern scholars have routinely dismissed a mid-Byzantine, anonymous account of abundant animal sacrifices allegedly performed by Emperor Justinian in 537 at the consecration of Hagia Sophia in Constantinople as disturbingly false and shocking, considering its Christian context: it is a well-known fact, the argument goes, that Christianity had outlawed such practices. Even if we embrace the scholarly consensus that no animals were ritually slaughtered at the encaenia of the Great Church, however, the fact remains that the very idea of animal sacrifice was not unknown to the Byzantines, quite to the contrary. This talk aims to pull together the evidence from late antique and medieval sources that bear witness to a widespread, ongoing practice of killing animals within a Christian ritual framework. Apart from the well-known Armenian tradition of animal sacrifice (matat), which continues to be practised to date, “Christianized” ritual slaughter of animals was also performed in other corners of both the Latin West and the Greek East. Despite being relegated to the margins of mainstream ritual practice and in breach of recurrent bans by official doctrine, sacrificial offerings of animals continued to be performed with the connivance of ecclesiastic officials and sometimes even occurred under their ceremonial leadership. A “Christianised” form of the popular ritual of animal sacrifice, however, was to be purified from its Pagan and Jewish, expiatory or appeasing, functions, and to be infused instead with a new meaning of communion with and gratitude to the only true Christian God.