1. Genesis 1:1 In the beginning, God created the heavens and the earth.
Genesis 1:26 Then God said, "Let us make man in our image, after our likeness. And let them have dominion over the fish of the sea and over the birds of the heavens and over the livestock and over all the earth and over every creeping thing that creeps on the earth."
el-o-heem‘ Plural of H433 (el-o-ah); “gods in the ordinary sense; but specifically used (in the plural thus, especially with the article) of the supreme God; occasionally applied by way of deference to magistrates; and sometimes as a superlative) (Strong)
It is the idea of deities or Deities.

2. But the Bible says there is One God
Mark 12:29 Jesus answered, "The most important is, 'Hear, O Israel: The Lord our God, the Lord is one.
Mark 12:32 And the scribe said to him, "You are right, Teacher. You have truly said that he is one, and there is no other besides him.
Lord – “koo'-ree-os From κῦρος kuros (supremacy); supreme in authority” (Strongs)
God – “theh'-os Of uncertain affinity; a deity, especially (with G3588) the supreme Divinity”

3. The Bible speaks of all Three together
Matthew 28:19 Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit,
Matthew 3: And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: (17) And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.

4. The concept of ONE in nature
Genesis 2:20 But for Adam there was not found a helper fit for him.
Genesis 2: Then the man said, "This at last is bone of my bones and flesh of my flesh; she shall be called Woman, because she was taken out of Man." (24) Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh.
There was no one suitable or like him, no one to be his counterpart or equal who could share his thoughts, joys, and dreams. There was no one who could think, reason, and understand; in other words, there was no one with his nature.
“This at last is bone of my bones and flesh of my flesh” speaks to God resolving man’s need for companionship by creating a suitable companion.
Not every man and woman become “one flesh”.
The one flesh is in context with marriage and companionship in marriage.
One God could speak to one Devine Nature as there is one human nature.
There is one God, Deity just as there is one man, humankind.

5. The concept of ONE in nature
Possesses the attributes of Deity.
Eternal
Deut. 33:27
Rev. 1:8
Heb. 9:14
Deuteronomy 33:27 The eternal God is your dwelling place, and underneath are the everlasting arms. And he thrust out the enemy before you and said, 'Destroy.‘
Revelation 1:8 "I am the Alpha and the Omega," says the Lord God, "who is and who was and who is to come, the Almighty."
Hebrews 9:14 how much more will the blood of Christ, who through the eternal Spirit offered himself without blemish to God, purify our conscience from dead works to serve the living God.

6. The concept of ONE in purpose
John 17: that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me. (22) The glory that you have given me I have given to them, that they may be one even as we are one, (23) I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me.
We can become one, in the manner of this context, only in the Father and Son.
This is how there can be One God, but three Persons.

7. The concept of ONE in purpose
Eternal purpose – Eph. 3:11
Jesus – John 5:30
Holy Spirit – John 16:26
Ephesians 3:11 This was according to the eternal purpose that he has realized in Christ Jesus our Lord,
John 5:30 "I can do nothing on my own. As I hear, I judge, and my judgment is just, because I seek not my own will but the will of him who sent me.
John 14:26 But the Helper, the Holy Spirit, whom the Father will send in my name, he will teach you all things and bring to your remembrance all that I have said to you.

8. The concept of ONE in work
The Father’s plan – Acts 2:23
Christ executed the plan – John 19:30
The Holy Spirit revealed the plan – 2 Pet. 1:20-21
Acts 2:23 this Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men.
John 19:30 When Jesus had received the sour wine, he said, "It is finished," and he bowed his head and gave up his spirit.
2 Peter 1: knowing this first of all, that no prophecy of Scripture comes from someone's own interpretation. (21) For no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit.
Mark 16: And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues; (18) they will pick up serpents with their hands; and if they drink any deadly poison, it will not hurt them; they will lay their hands on the sick, and they will recover." (19) So then the Lord Jesus, after he had spoken to them, was taken up into heaven and sat down at the right hand of God. (20) And they went out and preached everywhere, while the Lord worked with them and confirmed the message by accompanying signs.