Is 'Existentialism' just about pessimism, hopelessness and despair?

"Existentialism is nothing else but an attempt to draw conclusions from a consistently atheistic position. Its intention is not in the least that of plunging men into despair. And if by despair one means--as the Christians do--any attitude of unbelief, the despair of the existentialists is something very different. Existentialism is not atheist in the sense that it would exhaust itself in demonstrations of the non-existence of God. It declares, rather, that even if God existed that would make no difference from its point of view. Not that we believe God does exist, but we think that the real problem is not that of His existence; what man needs is to find himself again and to understand that nothing can save him from himself, not even a valid proof of the existence of God. In this sense existentialism is optimistic, it is a doctrine of action and it is only by self-deception, by confusing their own despair with ours that Christians can describe us as without hope."
--“Existentialism and humanism” (1948), Jean-Paul Sartre