Unitarianism, as opposed to Trinitarianism, is the view that divine sovereignty is not shared in any way.
Effectively, in unitarian views, there is not only just one God, but He is only revealed through just one person. In Trinitarian Christianity, the one God is revealed through three persons, the Father, the Son, and the Holy Spirit.
Given that atheism is clearly false, and that polytheism is also clearly false, and that therefore monotheism is clearly true, it may seem to many people that unitarianism must also be true. However, unitarianism is not the same thing as monotheism. Again, in all unitarian views, the key is that divine sovereignty is not shared in any way. But how can we see that this is false?
There are at least two ways we can see that unitarianism is false. First, deductively.
Any rational, sovereign, omniscient author of time and the universe -- any monotheistic God -- will, of necessity, eternally conceive of Himself as his own ultimate reason for everything that occurs. The conception of Himself will eternally be the exact representation of Himself, sharing divine sovereignty, since this conception is the reason for everything that occurs.
You see, God and His conception or image of Himself are both truly God. And both will exist if God is rational. And yet, there is a distinction between the concept and the thing conceived of. Thus, of deductive necessity, divine sovereignty will certainly be shared by God and God's reason, which is none other than God Himself.
Deduction therefore necessitates that unitarianism is false.
But secondly, the Bible, in agreement with deduction, also makes this clear.
In the beginning was the Word (or reason), and the Word (or reason) was with God, and the Word (or reason) was God. He was in the beginning with God. All things came into being through Him, and apart from Him nothing came into being that has come into being.
- John 1:1-3
He is the radiance of His glory and the exact representation of His nature, and upholds all things by the word of His power.
- Hebrews 1:3
"He who has seen Me has seen the Father."
- John 14:9
Unitarianism is thus both Biblically and also deductively false.