Reddit mentions of Once Before Time: A Whole Story of the Universe

Well, for starters, there is no "before" the Big Bang according to that theory, since time itself is a consequence of it. There is still the problem of what caused it, yes, and for that there is no commonly held theory (although there are many hypothesis, such as loop quantum cosmology, also see here), since scientists can't just make stuff up as true, and they don't have any strong evidence for any of those hypothesis yet. Again, I want to point out there is no "belief" in any of those hypothesis. Not even from the people who proposed them. They are acknowledge as fallible.

But perceiving your implied point, let me put it to you this way: if you use the notion of a god to explain the causality of the Big Bang, then what caused the existence of that god? What was there "before" it existed? Did it will itself into being? Does that make more sense?

Also, because it's impossible to prove the nonexistence or nonoccurrence of an abstract concept, we can't prove that god didn't cause the Big Bang any more than we can't prove that Charlie the Unicorn didn't cause the Big Bang, or that none of this is actually real and you're just a brain in a vat. It doesn't mean we should believe in any of those hypothesis.