These things Jesus spoke: and lifting up His eyes to Heaven, He said: "The hour has come. Glorify Your Son, that Your Son may glorify You. As You have given Him power over all flesh, that He may give eternal life to all whom You have given Him. Now this is eternal life: That they may know You, the only true God, and Jesus Christ, Whom You have sent."The sense is not that Jesus Christ is not true God, for the Scriptures clearly declare that Jesus Christ is true God. The same St. John declares [1 John 5:20], "And we know that the Son of God has come. And He has given us understanding that we may know the true God and may be in His true Son. This is the true God and life eternal." All the Fathers who commented on this passage understand it to be a proof of the divinity of Christ (Drum). See, e.g., St. Athanasius the Great [Statement of Faith 1; Letter to the Bishops of Egypt and Libya 2:13], St. Augustine the Great [Sermon 90:6 on the New Testament], and St. Gregory the Great [Letter XI:67 to Bishop Quiricus]. The NAB therefore justly translates the Johannine verse as "He is the true God and eternal life."
John 17:3 does not mean that "an exclusive diction can be joined to the personal term" when we speak of God, as St. Thomas Aquinas explains [ST I, q. 31, art. 4, ad 1]:
When we say, "Thee the only true God," we do not understand it as referring to the person of the Father, but to the whole Trinity, as Augustine expounds (De Trin. vi, 9). Or, if understood of the person of the Father, the other persons are not excluded by reason of the unity of essence; in so far as the word "only" excludes another thing, as above explained.Here is how several glorious Church Fathers explain the mystery. From the East: Sts. Athanasius the Great [Discourse 3:24:7-9 Against the Arians], Gregory the Theologian [Oration 30:13], John Chrysostom the Great [Homilies 4:2, 31:1, and 80:2 on the Gospel of St. John]. From the West: Sts. Irenaeus [Against Heresies 4:1:2], Cyprian the Martyr [Letter 72:17], Hilary of Poitiers [On the Trinity 9:29,36], Ambrose the Great [On the Holy Spirit 2:3:26-28; Exposition of the Christian Faith 5:1:16-26], and Augustine the Great [On the Trinity 6:9:10].
Bishop St. Irenaeus of Lyons, Against Heresies 4:1:2 (A.D. 180):
Now to whom is it not clear, that if the Lord had known many fathers and gods, He would not have taught His disciples to know [only] one God [John 17:3] and to call Him alone Father? But He did the rather distinguish those who by word merely (verbo tenus) are termed gods, from Him Who is truly God, that they should not err as to His doctrine, nor understand one [in mistake] for another.Bishop St. Cyprian the Martyr of Carthage, Letter 72:17 to Jubaianus Concerning the Baptism of Heretics (A.D. 255):
For they who know God the Father the Creator, ought also to know Christ the Son, lest they should flatter and applaud themselves about the Father alone, without the acknowledgment of His Son, Who also said, "No man comes to the Father but by Me" [John 14:6]. But He, the same, sets forth, that it is the knowledge of the two which saves, when He says, "And this is life eternal, that they might know You, the only true God, and Jesus Christ, Whom You have sent" [John 17:3] ... therefore, from the preaching and testimony of Christ Himself, the Father Who sent must be first known, then afterwards Christ, Who was sent, and there cannot be a hope of salvation except by knowing the two together.Archbishop St. John Chrysostom the Great of Constantinople (Doctor), Homily 4:2 on the Gospel of St. John:
And what mean the expressions, "I am the first and I am the last" [Isaiah 44:6]; and, "before Me was no other God formed" [Isaiah 43:10]? For if the Son be not of the same Essence, there is another God; and if He be not Co-eternal, He is after Him; and if He did not proceed from His Essence, clear it is that He was made. But if they assert, that these things were said to distinguish Him from idols, why do they not allow that it is to distinguish Him from idols that He says, "the Only True God" [John 17:3]? Besides, if this was said to distinguish Him from idols, how would you interpret the whole sentence? "After Me," He says, "is no other God" [Isaiah 43:10]. In saying this, He does not exclude the Son, but that After Me there is no idol God, not that there is no Son.He says in Homily 80:2 on the Gospel of St. John:
"The only true God," He says, by way of distinction from those which are not gods; for He was about to send them to the Gentiles. But if they will not allow this, but on account of this word "only" reject the Son from being true God, in this way as they proceed they reject Him from being God at all. For He also says, "You seek not the glory which is from the only God" [John 5:44]. Well then; shall not the Son be God? But if the Son be God, and the Son of the Father Who is called the Only God, it is clear that He also is true, and the Son of Him Who is called the Only true God. Why, when Paul says, "Or I only and Barnabas" [1 Corinthians 9:6], does he exclude Barnabas? Not at all; for the "only" is put by way of distinction from others. And, if He be not true God, how is He Truth? For truth far surpasses what is true. What shall we call the not being a true man, tell me? Shall we not call it the not being a man at all? So if the Son is not true God, how is He God? And how makes He us gods and sons, if He is not true?