Luca Mombello, “Immaculata and God the Father,” c. 1570 (Public Domain)
“There exists but one God ... he is the Father, God, the Creator, the author, the giver of order.” —St. Irenaeus
Some misguided folks actually claim that the Old Testament, and even the New Testament, taught “henotheism” — which is a form of polytheism holding that one God only is worshiped, but the existence of other gods is acknowledged. This is untrue. Monotheism (one God and one God only) is crystal clear throughout the Bible:
Exodus 8:10 (RSV). . . there is no one like the LORD our God. (cf. 1 Chr 17:20; 2 Chr 6:14)
Deuteronomy 4:35, 39 To you it was shown, that you might know that the LORD is God; there is no other besides him. . . .  . . . the LORD is God in heaven above and on the earth beneath; there is no other. (cf. Neh 9:6; 2 Sam 7:22; 1 Kgs 8:23; Is 43:10; 44:6, 8; 45:5-6, 14, 18, 21-22; 46:9; 47:8; Hos 13:4)
Deuteronomy 6:4 Hear, O Israel: The LORD our God is one LORD; (cf. Mk 12:29; Jas 2:19)
Deuteronomy 32:39 See now that I, even I, am he, and there is no god beside me; I kill and I make alive; I wound and I heal; and there is none that can deliver out of my hand. (cf. Ps 83:18; 86:10)
Isaiah 37:20 …thou alone art the LORD. (cf. 37:16)
Isaiah 43:10 …Before me no god was formed, nor shall there be any after me.
Isaiah 44:8 …Is there a God besides me? There is no Rock; I know not any. (cf. 44:6; 45:5-6, 21-22; 46:9; Mal 2:10)
Ephesians 4:5-6 one Lord, one faith, one baptism,  one God and Father of us all, who is above all and through all and in all.
1 Timothy 2:5 For there is one God, . . .
The Bible also explains beyond any doubt that the other “gods” often referred to are not real (only imagined to be by false worshipers). The idea here is that they exist in other religious belief-systems: but not in fact or reality. They are merely carvings of wood and stone, and have no conscious existence:
Genesis 31:19 . . . Rachel stole her father’s household gods. (cf. 35:2, 4)
Exodus 20:23 You shall not make gods of silver to be with me, nor shall you make for yourselves gods of gold. (cf. Lev 19:4)
Deuteronomy 4:27-28 And the LORD will scatter you among the peoples, and you will be left few in number among the nations where the LORD will drive you.  And there you will serve gods of wood and stone, the work of men’s hands, that neither see, nor hear, nor eat, nor smell. (cf. 29:17; 1 Chr 16:26; Is 2:8, 20; 31:7; 37:19; 44:10, 15, 17; 46:6; Jer 1:16)
Deuteronomy 28:36 The LORD will bring you, . . . to a nation that neither you nor your fathers have known; and there you shall serve other gods, of wood and stone. (cf. 28:64: “other gods, of wood and stone”)
Deuteronomy 32:17 They sacrificed to demons which were no gods, . . .
2 Kings 19:15-19 And Hezeki’ah prayed before the LORD, and said: “O LORD the God of Israel, who art enthroned above the cherubim, thou art the God, thou alone, of all the kingdoms of the earth; thou hast made heaven and earth.  Incline thy ear, O LORD, and hear; open thy eyes, O LORD, and see; and hear the words of Sennach’erib, which he has sent to mock the living God.  Of a truth, O LORD, the kings of Assyria have laid waste the nations and their lands,  and have cast their gods into the fire; for they were no gods, but the work of men’s hands, wood and stone; therefore they were destroyed.  So now, O LORD our God, save us, I beseech thee, from his hand, that all the kingdoms of the earth may know that thou, O LORD, art God alone.” (cf. Is 37:19; 45:20)
2 Chronicles 13:9 . . . Whoever comes to consecrate himself with a young bull or seven rams becomes a priest of what are no gods.
Psalm 40:4 . . . those who go astray after false gods!
Psalm 96:5 For all the gods of the peoples are idols; but the LORD made the heavens. (cf. 97:7; 115:4; 135:15)
Isaiah 37:19 …for they were no gods, but the work of men’s hands, wood and stone.…
Isaiah 44:10 Who fashions a god or casts an image, that is profitable for nothing?
Isaiah 44:15 . . . he makes a god and worships it, he makes it a graven image and falls down before it.
Isaiah 44:17 And the rest of it he makes into a god, his idol; and falls down to it and worships it; he prays to it and says, “Deliver me, for thou art my god!”
Isaiah 46:6 Those who lavish gold from the purse, and weigh out silver in the scales, hire a goldsmith, and he makes it into a god; then they fall down and worship!
Jeremiah 2:11 Has a nation changed its gods, even though they are no gods? . . .
Jeremiah 5:7 . . . Your children have forsaken me, and have sworn by those who are no gods. . . . (cf. 10:14)
Jeremiah 16:20 Can man make for himself gods? Such are no gods! (cf. 51:17)
1 Corinthians 8:4-6 Hence, as to the eating of food offered to idols, we know that “an idol has no real existence,” and that “there is no God but one.”  For although there may be so-called gods in heaven or on earth — as indeed there are many “gods” and many “lords” —  yet for us there is one God, the Father, from whom are all things and for whom we exist, and one Lord, Jesus Christ, through whom are all things and through whom we exist.
Galatians 4:8 Formerly, when you did not know God, you were in bondage to beings that by nature are no gods.