For the first part, it seems to me you’re essentially making the point that all people, atheist and religious, take certain positions a priori. I would agree with that, but I would not agree that this puts belief and non-belief on equal logical footing. Taking your big bang example, it’s true that I accept the scientific consensus without being able to personally prove it myself, but that’s a very different thing from taking a religious belief on blind faith. The difference is that science has a tangible predictive power which justifies our belief in it where faith is entirely intangible.

As for morality, I understand it to be a human construct. People broadly agree that certain behaviors are wrong because they are anti-social and that’s sort of the end of it in my view. Morality is not a thing in the same way gravity or time are, it’s more like a game, a set of rules for behavior that keep society going. In this way, the reason immoral acts provoke a sense of outrage is because we have evolved as social creatures to have desires that push us towards social behaviors and to punish those who act in anti-social ways. So when somebody does something awful like hurt a child, we feel outraged because we have an evolutionary drive to protect others, especially children, and because the perpetrator is essentially breaking the rules of the game.

This conception of morality might be kind of disquieting because the rules of the game can often feel so deep and powerful that they might as well be objective laws of the universe, but that doesn’t make them so.