The dogma in many religions is that nothing created God or at least, there's no way to know. He exists outside our time and space. So from our perspective, God has always existed. If you believe that free will is true, then he can see all outcomes of the universe at once. If you don't believe in true free will, he sees the single timeline at once, from the big bang to the heat death of the universe.

Personally, I don't believe in any religions. They were all developed before we had a much clearer idea of how our world works and there's just too much folklore mixed in with actual, yet primitive, philosophical discussions to abide by any of it. But like with the simulation theory, we have no way to know what exists outside our time and space so you shouldn't worry. If there is some bored extra dimensional teenager simulating our universe on their GPU, it ultimately doesn't matter because we'll never know.