While traditional Christian theologians, like St. Thomas Aquinas, saw the world as providing evidence of God's existence, and also thought that rational arguments a priori could establish the existence of God, Kierkegaard does not think that this is the case. But Kierkegaard's conclusion about this could just as easily be derived from Sartre's premises. After all, if the world is absurd, and everything we do is absurd anyway, why not do the most absurd thing imaginable? And what could be more absurd than to believe in God? So why not? The atheists don't have any reason to believe in anything else, or really even to disbelieve in that, so we may as well go for it!
- Kelley L. Ross, in the "Existentialism" article at The Proceedings of the Friesian School