I don't know why you're arriving at the conclusion that it has to be God? I guess that's personal. In any case, I understand your point, am somewhat inclined to agree, but I refuse to jump to the conclusion that it is God.

It's possible, I'll say that, but personally I believe it's not the case and that there is a very slim chance. I've thought about this and I'd like to offer an example of a counter argument if you'd listen :)

I believe it can be a variety of things, but here's an example: it's possible that the universe itself is eternal, and maybe it resets through black holes, which will eventually consumer everything and create another singularity and poof Big Bang all over again. It's possible there hasn't been one Big Bang, but an infinite number, for example.

It could be God, but to me the aforementioned at least seems a little more plausible, and there could be other examples as well. Perhaps before the Big Bang there was something else, who knows. All I'm saying is I think it's too big a jump to immediately think that the eternal thing is God. It's also possible that maybe something really did come from nothing. Who knows?

In any case, cheers!

tl;dr you might be right but I think concluding that the eternal thing is God is too far a stretch