If God is perfect, he cannot change. After all, if he would, he would be imperfect either before or after that change. Now if God cannot change, how can he have created the universe? This was more or less the argument that Aristotle used to prove that the world was not created and, therefore, had to be eternal.
It’s a good point and it took centuries before it was refuted. The most famous treatment is by Augustine of Hippo, who addresses the question what God did before the creation in the Confessions. After having joked that before the creation, God created Hell for people who ask improper questions, he made the simple but profound point that if God created everything, time had been created as well, and that there was no “before” before the creation. Aristotle’s question was devoid of meaning.