No I wasn't saying that all Existentialists should be atheists, I meant to say that basically Existentialism is based on the idea that there is that "nightside" of my mind or soul, and that I cannot get rid of it. Moreover, nobody except me personally can access this "nightside", nobody except me knows about it, so I'm absolutely along in front of it, I have to make all decisions on my own, nobody will help me simply because nobody can help me. This contradicts this view I called "classic Christian view" because, obviously, Christians believe that 1) God can access this "nightside", 2) God can help me to get rid of it, because 3) it is possible to get rid of this "nightside". These are three points where Christianity contradicts Existentialism. Moreover, say, Calvinist Christians believe that I don't have access to this nightside at all, that only God can see and understand it, which is totally incompatible with Existentialist basics.

Kirkegaard, is, again, somehow compatible with Existentialism because his concept of God is not that of a person who gives laws and judges, or suffers on the cross, Kierkegaard's God is like the impersonal abyss inside us, it's not transcendental, it's immanent. One may even say that Kiekegaard's God is himself inside the nightside of our soul, God is sitting at the very root of it, causing nausea and disbalance. It's somehow compatible with Existentialism though not fully.

And Dostoyevsky has very traditional concept of God and Human. God is all-seeing, all-knowing transcendental Person, human is erring being trying to find its way to purity. There's no Existentialism here, as far as I understand it.