God is outside of time. He created time with all other things. The mind cannot concieve what exactly or how exactly eternity is. God just 'is'.
It should have always existed. What held him back? If something held him back, he's limiting his omnipotence. If he's existed for eternity and is all-knowing, he's known for eternity that he was going to create the universe, so therefore the universe should have existed for all eternity aswell, but this is illogical because it would mean he couldn't have created it. It would have always been there. So either God hasn't existed for eternity, both God and the universe have an infinite existence and both weren't created or God doesn't exist and the universe is finite and was created by itself