I looked around a bit and while Oscar Isaac is Christian and not Jewish, his family name, Isaac, is Jewish on his father's side. His parents are from Cuba and Honduras, I think it was, and while small, there is a Jewish segment among Hispanics. So for Marc's family to look like the actor and still be Jewish, and at some point probably fled persecution to come to the US, is not a huge stretch. Now, I don't know if we will ever find out the decision by the writers and director to make Marc Jewish, but seeing so much of his past family life, I would like to think it had a reason other than just being different than the usual Christian, and mainly Catholic, stuff that we get when dramas include religion.