How did the first person figure out that God existed? Before there was a Bible or Koran or other scripture to tell us, what was it about the world that tipped us off?
It may have been something miraculous like the Voice coming from the burning bush. Exodus 3: 1-4. Or we might have been persuaded by the deeply moving splendor and complexity of life and nature. These might account for our belief in God's omniscience and omnipotence, but they would not explain why we came to believe God to be benevolent.
The candidate I would propose is the tendency for things to work themselves out better when we accept them as they are than when we try to change them to suit our preferences. The religious now know this as faith: that we are better off letting God govern what happens to us than we are when we try to change what we don't like. (Those unfamiliar with this often dismiss it as the subjective delusion of the already-persuaded religious who see only what agrees with their beliefs: More on that in another post.)
The pre-scriptural ancients would have been able to account for this only by positing an omniscient, omnipotent and benevolent being operating outside physical reality who can be relied on to arrange events to our benefit, but only when we get out of the way and accept whatever does or doesn't happen.
The young would never expect this and so would continually explain away why their efforts at changing and improving things are so often thwarted. But an older person reflecting on life might come to admit that trying to subdue the world is a fool's errand and doomed to frustration, whereas somehow, problems that are left alone will inevitably work themselves out.
To explain this, how reality can be so benevolent when left alone, the ancient might have concluded that there must be Someone who oversees everything (omniscient and omnipotent) and who will work everything out to our benefit when we resist the urge to interfere: ie, God.
See Jacob's Mirror: A Reconciliation of Science and Religion on YouTube or at Amazon. #scienceandreligion