Existentialism is about being a saint without God; being your own hero, without all the sanction and support of religion or society.
Existentialism is possible only in a world where God is dead or a luxury, and where Christianity is dead.
We were essentially torn from the Gaian womb, thrust into the birth canal of history, and expelled sometime around the fall of the Roman Empire into the cold hard world of modern science, existentialism and all the rest of it.