Why Mark Starts With a Virgin Birth in Matthew and Luke

Matthew and Luke are the only two gospels that have a virgin birth. None of the rest of the New Testament mentions Jesus being born of a virgin. And so the question is, why doesn't Mark start there? My guess for one thing is that Mark maybe didn't hear about it. This was not a story he was familiar with. But also, I think thatMark wants to emphasize that Jesus was the one chosen by God to do his will. He's the son of God in a way that's completely different from what anybody would have expected.