When rejecting a belief in God, the atheist must either ignore and not learn anything to replace their belief, or they must move on to the logical consequence of their lack of belief, namely that something came from nothing or else the universe is eternal and has had an eternal series of big bangs prior to the most recent.
Or we can just be really honest with ourselves and say, "Gee, I just don't know."
Your use of 'must' is also most unusual.