Nineteen, you don't seem to appreciate what Existentialism is in broad terms (or rather you have only encoutered one subset, which is fair enough). It is not an anti-religious philosophy necessarily.
Check out some Kierkegaard - a religious existentialist. He is probably at the "opposite" end of the scale to the pugnacious nihilism of a Nietzsche or Sartre. What existentialists tend to have in common is a sense that all value is subjective. You might find this easier to reconcile with a god-concept, than the thiestic hand-me-down system of morality adopted by traditional Western religions.
FWIW - I find absolutely no reason to believe in sentient "spiritual" beings of any sort. Just seems the most unlikely explanation for anything to me.