“I am the Alpha and the Omega,” says the Lord God, “who is and who was and who is to come, the Almighty.” -Revelation 1:8
That is God the Father speaking. He calls himself the Alpha and the Omega.
In the Old Testament:
Thus says the Lord, the King of Israel
and his Redeemer, the Lord of hosts:
“I am the first and I am the last;
besides me there is no god.”
God calls himself the first and the last. He is declaring his eternal nature.
He says elsewhere, “Before me no god was formed, nor shall there be any after me.” (Isaiah 43:10)
Yahweh is the beginning and the end. There is no other god, there never has been any other god, just the one. “Hear, O Israel: The Lord our God, the Lord is one.”
There is tons of evidence that Jesus Christ is God in the flesh–he’s already called himself “I AM” and he’s been called “God” and “Lord” many times.
Yet at the very end of the Bible he declared something that can be said of no one but God. Jesus said, as God the Father already declared to John the Revelator earlier:
“Behold, I am coming soon… I am the Alpha and the Omega, the first and the last, the beginning and the end.”
If it were not clear already, Jesus is declaring his oneness with the Father. There is no god other than God, so if another is also eternal, then they MUST be ONE.
As John Piper puts it in Seeing and Savoring Jesus Christ:
Jesus Christ is the Creator of the universe… the Alpha and Omega, the first and the last. Jesus Christ the Person, never had a beginning. He is absolute Reality. He has the unparalleled honor and unique glory of being there first and always. He never came into being. He was eternally begotten. The Father has eternally enjoyed “the radiance of the glory of God and the exact imprint of his nature” in the Person of his Son.