A benefit of Unitarian theology is that it is stated, explicitly, in the text of the Bible. It is not constructed via piecing together different (and often contested) passages from different authors, and then synthesizing them via clever and intricate philosophical formulations.
This is important, because the Biblical authors are competent, and able to clearly teach what they believe about God. This also makes the fact that none of them, at any point, gave even a brief description of the doctrine of the Trinity, let alone attempted to explain it, an indication that they did not believe it. Out of the abundance of the heart, the mouth speaks (Matthew 12:34, Luke 6:45). If the doctrine of the Trinity were true, it would be one of the most exciting, awe-inspiring revelations in human history. Those who knew of it would not be able to keep from proclaiming it, or at least puzzling over it, or proclaiming it a mystery, if they could not completely explain it, even under divine inspiration. Yet, not a single paragraph, or even a sentence, is spared for its description, in over 1,000 written pages of the Bible.
Because the Father is Yahweh in the Hebrew Bible (Deuteronomy 14:1, 32:5-6, Psalm 89:26, Isaiah 63:16, 64:8, Jeremiah 3:19-20, Malachi 2:10), many verses could be produced from there in which the Yahweh is taught to be the only God, and these could be considered concise, explicit statements of Unitarianism, as well. However, this article will highlight four New Testament texts (1 Timothy 2:5: 1 Corinthians 8:5-6, Ephesians 4:4-6, John 17:3), because they distinguish the Father from Jesus, while affirming that the Father alone is God, which is more relevant to the controversy between Trinitarian and Unitarian theologies specifically.
1 Timothy 2:5
5 For there is one God, and one mediator between God and men, the man Christ Jesus;
Above is an explicit description of the relationship between God, mankind who is in need of reconciliation (that is, sinful mankind), and the "man" Jesus. Jesus is said to be a mediator between mankind, and God, which indicates that He Himself is not in need of mediation, nor is He God, but instead, a mediator; a third party, who makes intercession for the transgressors (Isaiah 53:12).
Consider how confusing this verse would be if the author believed that Jesus was Himself the "one God". Consider how it would need to be explained - "There is one God, and one mediator between God and men, (the one God, hypostatically united to a human person, Jesus Christ)". This same author had just written in the previous chapter that the "only God" is "invisible" (1 Timothy 1:17). Later, he writes of God, "It is He alone who has immortality, and dwells in unapproachable light, whom no one has ever seen or can see" (1 Timothy 6:16). He is not confused on who God is, nor is he being sneaky, or esoteric, in declaring what would appear to be contradictions, if in fact Jesus was God, without offering any explanations.
Instead, he clearly teaches that there is one God, and that God is distinct from Jesus, and is not Jesus, because Jesus, who is explicitly called a "man", without any qualification, is the mediator between the "one God" and sinful humanity. Jesus, in this passage, is a distinct subject from the "one God", meaning He is not the "one God". And, because there is only "one God", Jesus cannot be God. That is what this concise passage teaches.
1 Corinthians 8:5-6
5 For though there be that are called gods, whether in heaven or in earth, (as there be gods many, and lords many,)
6 But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him.
Above, the "one God" is explicitly identified as "the Father", and Jesus Christ, the "one Lord", is distinct from Him, meaning Jesus is not the "one God" of Christians. They occupy different positions - God, and Lord, with God being the "head" of the Lord (1 Corinthians 11:3) - above Him, and superior to Him (1 Corinthians 15:24-28). God does not have a "head" above Him, nor is He "subject" (1 Corinthians 15:28) to anyone. Jesus does, and is, because He is God's Messiah - the anointed of God - and not God Himself.
A theology which makes the "one God" and "one Lord" in this passage one and the same Being, is attributing a very confusing and apparently incoherent style to the author. And yet, this passage under such a reading would be a great paradox introduced completely incidentally, carried on throughout the book, and never once explained. This same author writes about the fact that "Christ belongs to God" (1 Corinthians 3:23). Are these the same Being? He writes that, "The Son himself will be subjected to the one who subjected all things to him" (1 Corinthians 15:28). Are these the same Being? There is no Biblical reason to come to such a conclusion. It is only later theological mandates which force one into such a position, which is then read into dozens of texts in which the inspired authors actually say things which appear disqualify the position.
Ephesians 4:4-6
4 There is one body, and one Spirit, even as ye are called in one hope of your calling;
5 One Lord, one faith, one baptism,
6 One God and Father of all, who is above all, and through all, and in you all.
Above very closely echoes 1 Corinthians 8:5-6, teaching that Christians have "one God", who is the "Father", and "one Lord", which can reasonably be assumed to refer to Jesus, given how this book associates Him with the title "Lord", in distinction from God (Ephesians 1:3, 1:17). The doctrine that the "one God" just is the Father is the essence of Unitarianism. Interestingly, the Son and Spirit are never called the "one God". It is always the Father, in every single instance in which it is clearly distinguished.
And, notice that the "Father of all" is "above all". Sometimes, there is an implied or contextual reason to assume that there are exceptions when the word "all" is used in Scripture, as it cannot always mean "every thing" in a universal way. However, here, the word "all" is used to describe how God is the "Father of all", which would, of course, include His beloved Son, Jesus Christ. Therefore, assuming that the word "all" that is used a few words later includes at least the same group that He is the Father of, when the passage says He is "above all", it would include Jesus Christ. Christians have "one God", the Father, above their "one Lord", Jesus Christ, who has been exalted to Lordship by His God.
John 17:3
3 And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
Above, Jesus calls the Father "the only true God", in distinction from Himself. If the Father is the "only true God", then it is also true that there are no others who are the "only true God". Therefore, the Father alone is God, which is what Unitarianism maintains is the proper Biblical concept of God.
Consider the reading of this verse in line with common Trinitarian theology, recalling that Jesus is the one speaking - "This is eternal life, that they may know (you, the same Being as me), the only true God, and (myself, also the only true God), who (you, the same Being as me) sent". Consider how much needs to be read into so many similar statements throughout Scripture. Is this sort of interpolation really what God and the authors of Scripture intended someone to understand, when reading the Bible? It is not more likely that such doctrines were never on the minds of the authors, and consequently they speak without embarrassment or explanation of how the Father, in distinction from the Son, is the God of Christians?
Unitarian theology, in distinction from Trinitarian theology, is taught explicitly in the Bible. There are verbatim statements teaching the central claim of Unitarians - that the one God just is the Father alone. These competent and inspired authors are able to teach with clarity on this subject, and their words compel readers to forgo dense and convoluted Trinitarian speculations, which require so much to be read into the various texts regarding God and Jesus - things which the authors themselves felt no need to explain.