Our pitiful human minds cannot begin to encompass a being as grandiose and beautiful as our God. He has always existed and will always exist. That is just something our minds cannot wrap around. To us, everything has a beginning and an end. God does not.
“Before the mountains were brought forth, or ever thou hadst formed the earth and the world, even from everlasting to everlasting, thou art God” (Psalm 90:2).
“Your throne is established from of old; Thou art from everlasting” (Psalm 93:2).