God is the ultimate authority in all matters. Who would know better than the Father himself if there are persons who share his Deity? For all who really believe in the Bible, the words of the God of the Bible should be definitive:
For I am God, and there is no one else; I am God and there is no one like me (Isa. 46:9).
There is no one other than me; I am the LORD, and there is no one else (Isa. 45:6).
True monotheism was not conceived by Israel. Rather, it originated with the God of Israel. His words are, “I am God and there is no one else; I am God and there is no one like me.” He leaves no room for other persons who share in his Deity. Neither does he say anything about having multiple personalities or manifestations. Consider these words in Isaiah:
For thus says the LORD — he who created the heavens, he is God, he who formed and made the earth, he established it; he did not create it to be void, but formed it to be filled with life — he says: “I am the LORD and there is no one else” (Isa. 45:18).
Six times in this verse God is spoken of in singular terms:
- He created the heavens
- He is God
- He formed and made the earth
- He established it
- He did not create it to be void
- He says “I am the LORD and there is no one else.”
If we are to believe in the Bible, then we must worship him as God and him only. In fact, God is addressed with singular pronouns and verbs many thousands of times in the Scriptures. We must not embrace any other person or persons as being Deity with him. We must not worship two or three and then say that “they” are the one God of the Bible. Neither can we rightly worship imagined personalities or supposed manifestations of God. To do so opposes the very words of God himself. He disallows all such reasoning:
I am the LORD, and there is no one else; other than me there is no God. I strengthen you, though you do not know me, so that all may know; from the east and from the west, that there is no one other than me; I am the LORD, and there is no one else (Isa. 45:5, 6).
If we let him decide, then the answer is clear: The new multi-person monotheism does not stand the test of God’s own words. The one who is the God of the Bible declares that apart from “me” there is no God. His words exclude any other from being God with him. And if the God of the Bible is not a Trinitarian, then why should I be?
Gill, J. Dan (2016). God is Not a Trinitarian. In, The One: In Defense of God (pp. 95-96). Nashville, TN: 21st Century Reformation Publishing.
 For a consideration of the four verses where it is sometimes argued that God is referred to with plural pronouns, see chapter 10 of this book. For an in-depth analysis of singular pronouns in the Bible with regard to God, see Dr. Dale Tuggy’s article, “Divine Deception and Monotheism,” Journal of Analytic Theology, Vol. 2, May 2014, http://journalofanalytictheology.com/jat/index.php/jat/article/ view/jat.2014 1.030004192024a/232.
 I have borrowed here (with adaptation) from the title of Anthony Buzzard’s book, Jesus Was Not a Trinitarian (Morrow, GA: Restoration Fellowship, 2007).