I think we sometimes underestimate how powerful human ideas are. We sometimes tend to neglect the ivory tower scholars who give their lives to thinking through weighty questions of philosophy by saying, "What does that have to do with the practical world in which I live?" I don't know of any philosophy in history, with the possible exception of Marxism, that has had such a radical impact, so widely and quickly, on the shaping of human culture than the philosophy of existentialism.
Existentialism has many different varieties to it. Its general theme is that it focuses concern on human existence. That's why is called existentialism. The concern is not upon some kind of abstract principle of metaphysics or of God or things of that nature but the concern is with people, human beings, and what does it mean to exist as a person in this world. Existentialism, in focusing on the predicament of a human being, tends to be pessimistic and atheistic, although there are religious forms of existentialism and more optimistic forms of existentialism.
But the bottom line is this - existentialism tends to see man in a mood or an atmosphere of despair. Two of the great contributors of 2Oth century existentialism were Albert Camus and Jean-Paul Sartre who were Frenchmen. They responded to the holocaust of World War II and their ideas were those of great despair. For example, they came to the conclusion that man in his existence is a useless passion and that human life, ultimately, is meaningless and insignificant.
Of course in this view with respect to the things of God, the idea is that in Heaven there is nobody home. I remember the Greenwich Village advocate of existentialism a couple of decades ago made this quip to a reporter from Time Magazine, "Hey, man, I looked God up in the yellow pages but He wasn't listed." The idea is that there is no one home in the universe and we are left here in our existence in an atmosphere of ultimate despair.
You ask, how shall we respond to it? One thing for which I'm grateful in terms of existentialism is that existentialism produces a tremendous fertile ground for the preaching of Christianity because Christianity is so optimistic. We belicve that human existence is meaningful and that it is ultimately meaningful because Christ has defined the significance of our existence.