Who is Joshua from Lamb: The Gospel According to Biff and what is their importance?

Joshua grows up in Nazareth with Biff, his childhood pal. Born the Son of God but not yet known as the Messiah, Joshua carries a heavy burden. He must somehow convince enough people that he is the Messiah, and he must learn the ancient wisdom that will make him the Messiah, a man capable of accepting an early death by the horribly painful method of crucifixion.