Atheism worships a dead god.

[Existentialist]

Neitzsche ... made the claim,viz that God was killed by the enlightenment. Sartre simply accepted the claim.

Sartre's position was more that if God does exist, it would't actually make any difference to us. The existentialist position is that since what defines an individual is his actions, which only occur as a result of the individual's free choice after his existence has been established, then even a valid proof of the existence of God wouldn't make any difference to the fundamental problem of being human, which is how to find, create and define the human being. The rationalistic atheist position is completely different from this. Rationalistic atheists are hostages to a not yet established proof of the existence of God. Thus when the rational atheist is asked, what would he or she do if they died, went to heaven and discovered that God actually exists after all, if they aren't avoiding the question on the grounds of its hypothetical nature, they tend to give a reply that suggests acceptance of the newly discovered subordination of humanity to the deity. The rationalistic atheist is therefore always subject to a risk that the existentialist deftly avoids, and is more vulnerable to replicating religious thinking from an atheistic position.