I think it’s important to understand that God is a universal concept that is intergenerationally transmitted. I wouldn’t have considered God whatsoever if my parents or someone from their generation hadn’t introduced the concept into my life – and I’m certain that the latter is representative of a huge majority of people
Even if it were the case that you were left to your own devices, you would have still invented some kind of rationale for certain inexplicable objects or circumstances in the world, but the probability of that rationale being the God of Christianity is very low. That “something bigger than us” is a Deus ex Machina, and does little for the advancement of the human being outside of spirituality – and we can get spiritual release and enlightenment from a number of other sources.
Moreover, I don’t think that you are taking into account the many cultures that practice Ancestor Worship, Obea or other forms of God worship that not only preclude Christianity and its doctrines, but do not take into consideration the value of the Christian God whatsoever. Your scenario is not universally applicable.