Existentialism isn't so atheistic that it wears itself out showing that God doesn't exist. Rather, it declares that even if God did exist, that would change nothing.
Fyodor Dostoevsky[to Jesus] You did not come down from the cross when they shouted to you, mocking and reviling you: "Come down from the cross and we will believe that it is you." You did not come down because, again, you did not want to enslave man by a miracle and thirsted for faith that is free, not miraculous...I swear, man is created weaker and baser than you thought him! How, how can he ever accomplish the same things as you? ...Respecting him less, you would have demanded less of him, and that would be closer to love, for his burden would be lighter.
Fyodor DostoevskyThough I do not believe in the order of things, still the sticky little leaves that come out in the spring are dear to me, the blue sky is dear to me, some people are dear to me, whom one loves sometimes, would you believe it, without even knowing why; some human deeds are dear to me, which one has perhaps long ceased believing in, but still honors with one's heart, out of old habit..." --Ivan Karamazov
Fyodor Dostoevsky