The Bible asserts that there exists a supreme Being. In the Bible, this supreme Being is given the title of God.
(Out of reverence, I choose to fully capitalize the title as GOD. This serves as a personal reminder to elevate God in my thinking.)
Certain Scriptures, such as Hebrews 11:6, directly affirm the existence of GOD.
6 And without faith it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him.
(Hebrews 11:6 NIV)
The first verse of the Bible affirms the existence of GOD:
1 In the beginning God made heaven and earth.
(Genesis 1:1 OSB)
While the Bible does affirm GOD's existence, it is not the focus of Scripture to lay out a comprehensive case for the existence of GOD. To be sure, a compelling case may be made from the Scriptures, but the writings that comprise the Bible were written at a time where most (but not all) people believed in supernatural gods.
Therefore, Scripture generally assumes that the reader believes GOD is real.
As the psalmist wrote:
1 The fool says in his heart, “There is no God.”
(Psalm 13 :1b OSB)