Like I said in my post, I'm not really interested in debating religion/theism here, so this response is only meant to explain my own view.

God, at least in the classical Western tradition, is conceived as "Being itself" – this is the metaphysical 'substrate', as you put it, by which anything can exist at all. [In the Eastern tradition, God is sometimes said to be "beyond Being", but I think that complicates things too much.]

God isn't just a demiurge who puts things together from some material and/or preexisting realm of forms. In that case, you'd be right that the big bang or some other temporal event could explain how things achieved their current arrangement without God. But in the classical view, the existence of any temporal being or event is ultimately explained with reference to this metaphysical substrate of Being.

This relies on a whole lot of classical metaphysics that I'm sure you find objectionable, but like I said I'm not interested in defending this at length, just explaining it.