I am the Alpha and the Omega, the beginning and the end. To the thirsty I will give from the fountain of the water of life without payment. He who conquers shall have this heritage, and I will be his God and he shall be my son. But as for the cowardly, the faithless, the polluted as for murderers, fornicators, sorcerers, idolaters, and all liars, their lot shall be in the lake that burns with fire and sulphur, which is the second death.
Alpha is the first letter of the Greek alphabet and omega is the last letter of the alphabet.
But God isn't speaking about alphabets. He is speaking of reality. God is absolutely the beginning and absolutely the end. Everything that is originates ultimately in him. And everything will somehow end with him.