Our God is eternal. Among many places in Scripture, we see this in Revelation 1:8, where John writes,
“I am the Alpha and the Omega,” says the Lord God, “who is and who was and who is to come, the Almighty.”
In the Greek alphabet, Alpha and Omega are the first and last letters. This is to say that God is the beginning, and He is the end.
John identifies Him here as "the Lord God." Not only is God divine, but He is Lord, in command of all things. There are some ideas of false gods who do not exercise lordship. Deists formulate a god who created but does not govern. Other created gods have some lordship, but that lordship isn't absolute. Such gods can be defeated by others.
Our God, however, is Lord. And our Lord is the God who is the Alpha and the Omega.
He is the One "who is and who was and who is to come." God always has been, and He always will be.
Furthermore, He is the Almighty. He is omnipotent and all-powerful. This is the eternal God.
Some theorize that the universe is eternal, which is completely unbiblical and untrue. But people have to try to make sense of existence apart from God. They'd rather believe in an eternal universe than in an eternal God. However, something that is eternal is not the same thing as someone who is both eternal and almighty.
What's the point of saying here at the beginning of Revelation that God is eternal and almighty? It is His eternality and His omnipotence that give us comfort in times of trouble. The Alpha and Omega will do as He pleases, and what He pleases to do is good.
Revelation is about to unpack this eternal God's perfect will.