The origins of Sinaiticus are unclear. It could be one of the 50 Bibles that early church historian and Bishop of Caesarea Eusebius prepared for Constantine. If so, it somehow arrived at St. Catherine's, the monastery at the reported site of Moses' burning bush, perhaps as a gift from the 6th century emperor Justinian. In any case, Sinaiticus became one of the monastery's most important possessions.