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"And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent." - John 17:3 (KJV)
By itself, this verse seems to teach clearly that the Father is the only God, the Son is another person and therefore the Son is not God, as the text says. (You could read it with a particular emphasis: (…) the only true God, AND Jesus Christ)
But good hermeneutics requires that we look into the context of the verse, the Book, the author’s usage of terminology, the whole Bible,… Let’s look at the verses immediately following the verse we have quoted:
"I have glorified thee on the earth: I have finished the work which thou gavest me to do. And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was." - John 17:4-5 (KJV, emphasis mine)
In verse 4-5, Jesus asks His Father to glorify Himself: “(…) O Father, glorify thou me with thine own self with the glory which I had with thee before the world was”. It would be good for us to find out what this glory was, in order to understand our John 17:3 verse. As we shall see, a study of the Gospel of John will suffice. [After each Bible text, the meaning of the glory will be summarized in block quotes.]
"In the beginning was the Word, and the Word was with God, and the Word was a god. This one was in the beginning with God." - John 1:1-2 (NWT, emphasis mine. The translation of this verse, by the Jehovah's Witnesses, is probably favoured by Unitarians.) "In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God." - John 1:1-2 (KJV, emphasis mine)
In the first chapter of John’s Gospel, we find that Jesus the Logos, was with God (verse 1 and 2) and that He was God or a god (verse 1). It should be kept in mind, that in Greek, the subject of a sentence follows the article, so that by the article, you can find which word, in the sentence, is your subject.
Furthermore, claiming that because the article is not found before theos, means that Jesus is not God, is a Unitarian concept that just doesn’t work. In verse 18, for example, theos is clearly used as referring to the Father. Are we to conclude then, that the Father is also not God but (just) a god??
Ernest C. Colwell remarked (as described by John Bechtle): ‘To conform to standard Greek grammar. E.C. Colwell demonstrated in an article in the Journal of Biblical Literature in 1933 that it was normal practice to omit “the” in this type of sentence. John was simply using good grammar, and making it clear that he intended to say, “The Word was God” rather than “God was the Word,” a statement with some theological drawbacks. John constructed his sentence in the one way that would preserve proper grammar and sound doctrine, declaring that “the Word was God”.’ source: John Bechtle, ‘Should John 1:1 be translated, “The Word was God” or “The Word was a god”?’ (christiananswers). So, the best way of writing “the Word was God”, was by writing it the way John wrote it.
Therefore, Merrill C. Tenney (whose commentary on the Book of Revelation, I highly recommend) could state: ‘To say the absence of the article bespeaks of the non-absolute deity of the Word is sheer folly. There are many places in this Gospel where the anarthrous theos appears (e.g. 1:6, 12, 13, 18), and not once is the implication that this is referring to just “a god”.’ source: Merrill C. Tenney, “The Gospel of John” in The Expositor’s Bible Commentary (1981), p.30.
So, from the evidence, we can infer that the first chapter of the Gospel of John indicates that Jesus is God and not just a god. But we will continue our investigation, to see what the rest of the Gospel of John says, as we are looking for more evidence, concerning Christ’s glory, to make a final statement.
Being with the Father (verse 1), from before His own act of creation (verse 3), being begotten (not made) of the Father (verse 14) and being in the very bosom of the Father (verse 18), are part of what Jesus’s glory consists of, according to this chapter.
"But Jesus answered them, My Father worketh hitherto, and I work. Therefore the Jews sought the more to kill him, because he not only had broken the sabbath, but said also that God was his Father, making himself equal with God. (...) That all men should honour the Son, even as they honour the Father. He that honoureth not the Son honoureth not the Father which hath sent him." - John 5:17-18, 23 (KJV)
John, the evangelist, clearly describes that the Jews understood Jesus to claim that He is God, that He is equal to His Father and [verse 23:] expects the same honour as God the Father (“All men should honour the Son, even as they honour the Father.”) even claiming that if He doesn’t get that same honour, from men, they are not honouring the Father at all! (“[All men should honour the Son, even as they honour the Father.] He that honoureth not the Son honoureth not the Father.”). This, as William Birch noted, forms a problem for Unitarians. We will describe his argument in the next paragraph, under John 10.
Jesus’s glory consists of being able to equate His work with the Father’s work, being equal to the Father and being allowed to expect to receive the same glory from his human creatures.
"I and my Father are one. Then the Jews took up stones again to stone him. Jesus answered them, Many good works have I shewed you from my Father; for which of those works do ye stone me? The Jews answered him, saying, For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makest thyself God. (...) The Father is in me, and I in him. Therefore they sought again to take him: but he escaped out of their hand." - John 10: 30-33, 38b-39 (KJV)
“I and my Father are one.” and “The Father is in me, and I in Him.” are 2 claims that would have gotten Jesus killed, if the Father did not sovereignly intervene. In John 10, as in John 5, the Jews clearly understood that He was claiming to be God. He had plenty of time to offer His apologies, in case they misunderstood Him. Therefore, ‘Anti-Trinitarians are obligated to side with Jesus’ opponents and confess that He actually was a blasphemer; that is, if Jesus is not “one” with the Father, the second Person of the Trinity, who shares of the divine Essence.’ source: William Birch, “Dale Tuggy and Anti-Trinitarians: Consequences of Denying the Trinity” (williambirch). [Click here to read the full argument.]
Jesus’s glory consists of being one with His Father and His Father being one with Him and being in the Father and the Father being in Him, which denotes an ultimate bond of unity and being of the same essence with the Father.
If I meet you on 2 different occasions and the first time I tell you that I’ve got an apron, while the second time I tell you that I’ve got a chef’s hat. Would you then conclude that I only have a chef’s hat and not an apron? That would be a truly bizarre conclusion.
It is the same with the John 17:3 text. If Jesus and the author of the Gospel of John claim many times over that Jesus is divine, that He is equal to the Father and that He is in the Father, shouldn’t we then read John 17:3 – which does not necessarily try to show Christ’s divinity, as that is not the intend of that specific text – in light of those passages, instead of taking them out of the equation, and then saying that Jesus doesn’t claim to be God?
The Gospel of John is clear that if one truly believes what is written there, he could only conclude that Jesus is God, like the Father is God. Since He demands equal glory, eternal life is not only knowing the Father but also the Son (John 17:3), who is also God. Why Jesus calls the Father “the only true God”, might remain a mystery. One explanation could be that He is referring to the shema, as found in Deuteronomy 6:4 (“Hear, O Israel: The Lord is our God. The Lord is one!” (MEV)), which does not deny that there can be more Persons in the Godhead – on the contrary – but it only denies that there are Gods next to the Monotheistic God of Christianity (read this and this for more information on the idea behind the shema).