God
But God's existence is yet to be established. Babies are oblivious to other people's belief in god - that's why parents usually pass on their beliefs to their children.
You don't get ready-made, new-born true believers, do you? Form this it seems reasonable to conclude that neonates are without belief in god/gods and are therefore atheists.
I think I explained my position as well as I can here:
Since I don't think they entertain any belief at all, I don't think they believe in God and I don't think they disbelieve in God. They simply have no belief or opinion in the matter.