The Bible says in:

Revelation 22:13 “I am Alpha and Omega, the beginning and the end, the first and the last.”

Deuteronomy 33:27a “The eternal God is thy refuge, and underneath are the everlasting arms:…”

John 1:1-3 “In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him; and without him was not any thing made that was made.

God had no beginning. He is the beginning. Everything was made by God.

God is eternal: He has always been and always will be.

As humans, our minds are finite. Meaning they are limited. We are not capable of fully understanding God, because if we were, then He would not be God.

It is impossible for us to fully comprehend that God had no beginning, because each of us had a beginning. This is when we must have faith. We must simply trust God, when He says that He has always, and will always exist.