Who is Menahem from The Source; a Novel and what is their importance?

Menahem is born out of wedlock. His mother runs off and the stonecutter, Yohanen, raises the boy, but he can never be accepted as a Jew. The rabbis at Tveryl decide that if he is caught in a small theft, sold into slavery with a Hebrew family and married to a Hebrew slave, in five years when they are free he can be accepted into a congregation and his children will be Jews. He talks to the priest about the basilica and finds he can be accepted as a convert to Christianity and freely marry. He adopts the Christian name of Mark.