Logic can only say that the premise and the conclusion are consistent. But it can't know whether that premise is true or not, or even know where that premise came from. You'll need another premise to know whether that premise is true or not... and you'll need another premise... and it becomes an infinite regress.

So where does that premise come from? And the answer is a bit mysterious, or we could say it comes from "creativity". Saying that "God exists" or "Because of God" is a human answer that comes from human creativity. I mean it's possible that the entire human civilization could have gone without ever coming up with the idea of God. I'd suppose it has actually been that way for a long time. The idea of an almighty monotheistic God is a fairly new idea.

We could also say that we are working from the middle, and not from the start, because there will always be a premise prior to that premise. So there is no "absolutely fundamental premise" that we are working from. It's possible that we have yet to understand the premise prior to "God exists", such as "What is God?".