God never came into being. He Is, without beginning or end. And when we say that God is eternal, we don’t necessarily mean that he has existed for an infinite time in the past, as if there was such a thing as time before existent things other than God came into being and He simply sat there for infinite time before creating.
God is not another being alongside creation. He’s not just some existing thing that is older and more powerful than anything else. He is more like a foundation to the existence of everything else, the foundation of reality. If created reality is like a series of rivers, He’s not simply another river, but the origin spring of all rivers. I’m trying to convey the sense that He is something other, something prior (not just in time) to all other things. A source. A foundation, upon which everything else is seated.