monotheism
The belief that there is only one God. (Christianity, Judaism and Islam are all monotheistic religions.)

• From scripture:

“You are my witnesses,” declares the LORD,

“and my servant whom I have chosen,

that you may know and believe me

and understand that I am he.

Before me no god was formed,

nor shall there be any after me. (Isaiah 43:10 ESV)

• From the Belgic Confession:

Article 1: The Only God

We all believe in our hearts and confess with our mouths that there is a single and simple spiritual being, whom we call God — eternal, incomprehensible, invisible, unchangeable, infinite, almighty; completely wise, just, and good, and the overflowing source of all good.

• From The Christian Faith by Michael Horton:

Faith in the One God—Yahweh—arose not out of Greek speculation but out of God’s self-revelation to Israel. The same God who forbade idolatry was addressed by Jesus as “Father.” In fact, Jesus answered Satan’s temptation by reasserting Israel’s creed, the Shema: “You shall worship the Lord your God and him only shall you serve” (Mt 4:10, paraphrasing Dt 6:13). Jesus’ acts of healing are pointers to this God and to no other: “And they glorified the God of Israel’ (Mt 15:31)… . Turning Gentiles ‘from idols to serve the living and true God’ was as essential a part of the paostles’ message as it had been for the prophets (1Th 1:9; cf. 1Pe 4:3). Appearing before the Roman governor Felix, Paul entreated, “This I confess to you, that according to the Way, which they call a sect, I worship the God of our father, believing everything laid down by the Law and written in the Prophets, having a hop in God” (Ac 24:14—15). Included in that confession is “one God” (Eph 4:6).

• From What Is a Monotheistic Religion? at Blue Letter Bible:

The fact that a religion is monotheistic is not enough. Belief must not only be in one God, belief must be in the right God! James wrote.

You believe that there is one God. Good! Even the demons believe that ‑ and shudder (James 2:19).