Many people have faith in evolution, or some humanistic utopia, or some idealized humanistic philosophy, or in some "god" they created in their head. Why do some claim to believe in God but "fall away" later? They never had faith in God but in a "god" they created in their head.

"They went out from us, but they were not really of us; for if they were of us, they would have remained with us; but they went out, so that it would be manifested that they all are not of us." - 1 John 2:19


Those with dead faith never had faith in God.