That’s what the definition of God is— it’s our word to describe that which is uncreated.
Because we know from experience everything physical and material has a beginning and an end. There is a time before it existed, and there will be a time when it no longer exists. So in order for something to be around before anything else began, there needs to be something that has no beginning and end.
And so we call that something “God”.
Usually, with theists, you run into difficulties trying to define what that God’s role is after he’s created it. Does he wind up the universe like a watch, and then wander off to let it do its thing? Or is he actively involved in what goes on? In what way, and in what extent? Those are the things theists debate about.
For an atheist, who doesn’t believe in God, they’ll say, “The watch (universe) just exists. Because it has to exist. Because it does. But it’s not like it was made or anything.” But the theist says, “It’s a watch. Look at how it works. That’s not how randomness operates. I don’t just leave a pile of stuff on the table, and a gazillion years pass, and it magically assembles itself into a watch. Or, I don’t just leave a giant vacuum of nothingness, and it magically assembles itself into materials that assemble themselves into a watch.”
So, theists need to have an explanation about why the world around them makes sense, where it came from, how it is a gazillion times more complicated than any watch, and why we have such a fraaaaaagile little teeny planet in the massive coldness and emptiness of space, and how it came to be that it teems with living, breathing, thinking life. The only thing that makes sense to us as Catholics is that something greater than the universe decided that it would be nice if it existed, and then went about arranging for it to happen. But atheists say, “Of course it exists. Why wouldn’t it?”
