I've heard some people say that Christianity is polytheistic for various reasons. For instance, I've heard once or twice that the ancient Israelites worshiped other "gods" in addition to God Himself. And while there was a lot of idolatry in ancient Israel, people's actions are not a reliable way of determining facts - as I've said multiple times on here. The Bible proves time and time again that God is the only god, no matter what human beings did. God is not part of mythology, He is not changed by stories. God never had a wife. (Be careful not to make a treaty with those who live in the land where you are going, or they will be a snare among you. Break down their altars, smash their sacred stones and cut down their Asherah poles. - Exodus 34:12-13)
This post is going to list various verses that state, beyond doubt, that God's original message was clear, that He was, and is, the only true god in existence. I'll add some now. I'll probably find more to add later.
"I am the LORD, and there is no other; Besides Me there is no God."
- Isaiah 45:9a
"that people may know, from the rising of the sun and from the west, that
there is none besides me; I am the LORD, and there is no other."
- Isaiah 45:6
For this is what the LORD says . . . "I am the LORD,
and there is no other.
- Isaiah 45:18
And there is no God apart from me, a righteous God and a Savior; there is none but me.
- Isaiah 45:21
I am God, and there is no other.
- Isaiah 45:22b
I am God, and there is no other; I am God, and there is none like me.
- Isaiah 46:9
You were shown these things so that you might know that the LORD is God; besides him there is no other.
- Deuteronomy 4:35
Who has done this and carried it through, calling forth the generations
from the beginning? I, the LORD--with the first of them and with the
last--I am he."
- Isaiah 41:4
Acknowledge and take to heart this day that the LORD is God in heaven above and on the earth below. There is no other.
- Deuteronomy 4:39
This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent.
- John 17:3
"The most important [commandment]," answered Jesus, "is this: 'Hear, O Israel: The Lord our God, the Lord is one.
Before me no god was formed, nor will there be one after me.
- Isaiah 43:10
God bless and guide you all according to His perfect will! In Jesus' name. Amen.