Constantine the Jew (c. 850 – 26 December, after 886) was a Byzantine Christian monk and evangelist venerated as a saint within his monastic milieu and in Constantinople. Born to a Jewish family in Synada, Constantine excelled at Hebrew and the Old Testament from a young age. He was said to have been converted to Christianity as a youth by the power of the sign of the cross, which he had made in spontaneous imitation of a Christian merchant. His full conversion was gradual. It may be linked to the campaign undertaken by the Emperor Basil I (r. 867–886) to convert the Jews early in his reign. Constantine's family arranged a marriage for him, but on his wedding day he fled to the monastery of , where he was finally baptised.