We cannot find the word Trinity in the Bible. However, the Bible forces us to face the reality that there is only ONE God (Deuteronomy 6:4; Isaiah 45:18; Romans 3:30; James 2:19) but that only one God has THREE persons. "The English word 'Trinity' comes from Latin 'Trinitas', meaning 'the number three'." (www.wikipedia.org)
Genesis 1:26, Genesis 3:22, Genesis 11:6,7, and Isaiah 6:8, among others, use PLURAL pronouns to denote God. Let's take a closer look at Genesis 1:26, it says, "And 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 fowl of the air, and over the cattle, and overall the earth, and over every creeping thing that creepeth upon the earth.”
Perhaps the clearest scripture about the Trinity is Matthew 28:19 saying, "Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and the Holy Ghost:" Notice that the word "name" is in the singular form. This means that the Father, the Son, and the Holy Ghost shares one "name".
Some Jews argue that the concept of the Trinity is exclusive to the New Testament. However, a careful examination of scripture reveals the contrary. Remember that we already tackled old testament scriptures that use PLURAL PRONOUNS to denote God. Another scripture passage is Psalms 45:6,7. It says, “Thy throne, O God, is for ever and ever: the sceptre of thy kingdom is a right sceptre. Thou lovest righteousness, and hatest wickedness: therefore God, thy God, hath anointed thee with the oil of gladness above thy fellows.” We just read about a person named God who is anointed by another person who is also named God, that's at least 2 persons sharing the same name -God.
We know from the book of Matthew that there 3 persons that share one name, the Father, the Son (that's Jesus), and the Holy Ghost, can we read about these 3 persons in the old testament? Sure! Let's read Isaiah 48:12, 16. Verse 12 says, "Hearken unto me, O Jacob and Israel, my called; I am he; I am the first, I also am the last." Here we read about a person who calls himself "The First and the Last". Considering Revelations chapter 1, We know that this person is no other than Jesus himself. Now, let's read verse 16, "Come ye near unto me, hear ye this; I have not spoken in secret from the beginning; from the time that it was, there am I: and now the Lord GOD, and his Spirit, hath sent me." Here we read that that Jesus was sent by the Lord God -referring to the Father, and His Spirit -referring to the Holy Ghost.
If the Bible is teaching that God has 3 persons, is the Bible teaching polytheism? What is polytheism? Lexico.com defines Polytheism as "the belief in or worship of more than one god." Is the Bible teaching that there is more than one God? No! The Bible does not teach that there are 3 gods. Instead, the Bible teaches that there is only one God but that only one God has 3 persons.
Perhaps the best illustration that will help us to understand the concept of one God having 3 persons, the marriage relationship. Genesis 2:24, "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh. " Here we have ONE FLESH that has TWO persons, the man, and his wife. If 1+1=1 can be, 1+1+1=1 should also be. Every time I think about how one God can have three persons, it almost always causes some pain in my head. However, every time I think about this, I am reminded, that God's mathematics is beyond human mathematics, that God's mind is beyond mine, and that I am nothing compared to Him. A man trying to explain God is like an ant try to explain man.
When we see God, will we see three persons? Sure! In fact, we already had a sample of the 3 persons present in the same place at the same time. When Jesus was baptized, the Holy Spirit hovered over Him in the form of a dove while the Father spoke from heaven. Revelations chapter 4 and chapter 5 described God's 3 persons being in heaven, the Ancient of days (referring to the Father) was sitting on the throne, the Seven Lamps (referring to the Holy Ghost) burning before the throne, and the Lamb (referring to Jesus) appearing before the throne.
Is trinitarianism polytheism? Polytheism is believing in more than one God. Trinitarianism is believing in the ONLY ONE GOD that has 3 persons. The Bible teaches that there is only one God but that one God has three persons. When we see God, will we see three persons? Sure! God has three persons even in heaven.
*All scripture used are from the King James Version Bible (1611).