Godhead

the nature of God especially as existing in three persons — Merriam-Webster Dictionary

The word Trinity cannot be found in the Bible, but the truth of it can. While there's only one God, the Godhead consists of three distinct persons - the Father, Son, and Holy Spirit. All are equally omniscient, omnipotent, omnipresent, eternal, and unchanging, but each has unique functions. — Christianity.com

In theological studies, the term Godhead is used to refer to the concept of the Triune God, or one God in three Persons that include God the Father, God the Son, and God the Holy Spirit. — Got Questions Ministries

Forasmuch then as we are the offspring of God, we ought not to think that the Godhead theios is like unto gold, or silver, or stone, graven by art and man's device. — Acts 17:29 (KJV)

Therefore, since we are the offspring of God, we ought not to think that the Divine Nature is like gold or silver or stone, something shaped by art and man’s devising. — Acts 17:29 (NKJV)

For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead theiotēs, so that they are without excuse — Romans 1:20 (NKJV)

For in Him dwells all the fullness of the Godhead theotés bodily. — Colossians 2:9 (NKJV)

Echad is used nearly 1000 times in the OT, and almost always refers to a single numerical entity. There are times when it is used of a composite entity (Genesis 2:24). It functions just like the English word “one,” which can be used of single or composite entities, although it most often refers to a single, solitary thing. Only the context can determine how echad is being used. Given the rarity with which echad is used to refer to a composite entity, we should understand echad as referring to a single entity unless there are good contextual clues that warrant the uncommon meaning.

So יהוה said to Mosheh, “See, I have made you an elohim to Pharaoh, and Aharon your brother is your prophet. — Exodus 7:1 (TS2009)

“He who slaughters to an elohim, except to יהוה only, is put under the ban. — Exodus 22:20 (TS2009)

But Joash said to all who stood against him, “Will you contend for Baal? Or will you save him? Whoever contends for him shall be put to death by morning. If he is a god (elohim, let him contend for himself, because his altar has been broken down.” — Judges 6:31 (ESV)

Will you not possess what Chemosh your god (elohim gives you to possess? And all that the LORD (YHVH)) our God (elohim has dispossessed before us, we will possess. — Judges 11:24 (ESV)

And when the men of Ashdod saw how things were, they said, “The ark of the God (elohim) of Israel must not remain with us, for his hand is hard against us and against Dagon our god (elohim).” — 1 Samuel 5:7 (ESV)

The term "Elohim" means “supreme one” or “mighty one.” It is not only used of the one true God but is also used on occasion to refer to human rulers, judges, and even angels. If you saw one exhibiting supreme rule and expressed mighty power, the word you would use would be Elohim.

In the Ancient Near East, it was common to refer to the deity in the compound plural, and when speaking of an owner or master, it was often the rule to speak of him in such terms.

To give you just a few examples,

• Abraham’s servant speaks of his master in the plural in Genesis 24 (ʾadonim, literally, “lords”),
• Joseph speaks of his master Potiphar in the plural in Genesis 39, and
• David the king is spoken of as “lords” in 1 Kings 1:11.
• In Exodus 21, to translate literally and incorrectly, the law speaks of a slave and his masters (ʾadonim, referring to just one master),
• in Isaiah 19:4, the prophet tells Israel that God will hand them over to a cruel lord (Hebrew, ʾadonim qasheh, a plural noun with a singular adjective), and
• Isaiah 1:3 tells us that a donkey knows the feeding crib of its masters (baʾalim, referring to just one person; cf. the first half of the verse in which reference is made to an ox’s owner—in the singular).

These examples, which are really very common, show clearly that compound plurals were often used to speak of leaders, owners, masters, or kings. How much more then could similar expressions be used to speak of the Lord, the Master, the King, and the God."

— Answering Jewish objections to Jesus: Theological objections, Michael L. Brow, A Jew who converted to Christianity, Vol. 2, p9, 2000 AD

Then God said, “Let us make man in our image, after our likeness. — Genesis 1:26 (ESV)

So God created man in his own image, in the image of God he created him. — Genesis 1:27 (ESV)

• The most common objection to the “us” in Genesis 1:26 referring to angels is that Scripture attests that God made mankind. But God could easily have headed up a council with whom He conferred, and afterward did the work they decided upon.
• Another objection to this view is that God goes on to say “our image” after saying “let us” so one might question how angels are in the image of God. Since Adam in his pre-fallen state was without sin and in the image of God, it is perfectly reasonable to assume angels in God’s divine council, were also created in the image of God, and without sin. Therefore, it presents no problem to say that humans were created after the image of God (and subsequently angels).

— Spirit & Truth Fellowship International

I am the LORD, Who made all things, Who alone stretched out the heavens, Who by Myself spread out the earth; — Isaiah 44:24 (NRSV)

The plural of majesty does exist of nouns… but Genesis 1:26 is not about nouns — the issue is the verbal forms.

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. — Genesis 1:26 (ESV)

Then the LORD God said, “Behold, the man has become like one of us in knowing good and evil. Now, lest he reach out his hand and take also of the tree of life and eat, and live forever” — Genesis 3:22 (ESV)

... therefore the LORD God sent him out from the garden of Eden to work the ground from which he was taken. He drove out the man, and at the east of the garden of Eden he placed the cherubim and a flaming sword that turned every way to guard the way to the tree of life. — Genesis 3:23-24 (ESV)

Come, let us go down and there confuse their language, so that they may not understand one another’s speech. — Genesis 11:7 (ESV)

Furthermore, the pronouns in the Bible that refer to “God” are singular, and there are lots of them.

Singular pronouns include “I,” “my,” and “he.”

We would expect that the pronouns that refer to the “Father,” to Jesus, and to “the Holy Spirit” would be singular if there were a Trinity, but since the Trinity teaches that “God” is triune and consists of three “Persons,” that the pronouns associated with “God” would be plural. This is especially the case because according to Trinitarian doctrine, each “Person” in the triune God is individually omnipresent, individually all-knowing; individually all-powerful, and each individually has his own will, his own mind (which is why Jesus could say to the Father, “not my will but yours be done”). John 3:16 (REV) reads,

But if “God” were composed of three co-equal beings who each had their own mind and together agreed to send Christ, we would expect it to say,

The fact that the pronouns in the Bible refer to “God” as a singular being is evidence that there is no Trinity.

— Spirit & Truth Fellowship International