In this short post I will sum up the evidence from the Johannine corpus that the Father is said to be the only true God, not to the exclusion of the Son (or the Holy Spirit), but in perfect inseparable union with his beloved Son (and his eternal Spirit.)
The following text,
“When Jesus spoke these words, He lifted His eyes toward heaven and said: ‘Father, the hour has come. Glorify Your Son, that Your Son may also glorify You. As You have given Him authority over all flesh, He will give eternal life to all whom You have given Him. This is eternal life: that they may know You, the only true God, AND Jesus Christ, whom You have sent.’” John 17:1-3
Is seen by anti-Trinitarians as proof that neither Jesus nor the inspired biblical writers taught the Trinity.
In reality, this merely highlights the propensity of anti-Trinitarians to misinterpret the God-breathed Scriptures by wrenching texts out of their immediate and overall contexts.
After all, the same inspired writer who has our blessed Lord affirming the essential Deity of his Father is also the same one who began his God-breathed proclamation by identifying the Son as the uncreated Word of God who brought all creation into existence. John is also the one who testifies to the Son being the true God and eternal life whom the Father sent to reveal God to mankind:
“In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things were created through Him, and without Him nothing was created that was created. In Him was life, and the life was the light of mankind. The light shines in darkness, but the darkness has not overcome it… The true Light, which enlightens everyone, was coming into the world. He was in the world, and the world was created through Him, yet the world did not know Him… The Word became flesh and dwelt among us, and we saw His glory, the glory as the only Son of the Father, full of grace and truth… No one has seen God at any time. The only Son, who is at the Father’s side, has made Him known.” John 1:1-5, 9-10, 14, 18
“We also know that the Son of God has come and has given us understanding so that we may know the true God. We are in union with the one who is true, his Son Jesus the Messiah, who is the true God and eternal life.” 1 John 5:20 ISV
Compare how the following translations render the foregoing text:
“… united with his Son Yeshua the Messiah. He is the genuine God and eternal life.” CJB
“… and in his Son Jesus Christ. This is the real God and this is real, eternal life.” PHILLIPS
“… in his Son Jesus Christ. He is the true God and eternal life.” NABRE
“…in his Son Jesus Christ. This one is the true God and eternal life.” NET
“… and we are in him that is true, that is, in that his Son Jesus Christ, the same is that very God, and that eternal life.” GNV
“… in Jesus Christ his Son, who is the only true God; and he is eternal Life.” TLB
“… We are in the one who is real, his Son Jesus Christ. This Jesus Christ is the real God and eternal life.” GW
“… and we are living in the Truth itself, in God’s Son, Jesus Christ. This Jesus is both True God and Real Life.” MSG
“… We are in the one who is real, his Son Yeshua Christ. This Yeshua Christ is the real God and eternal life.” NOG
That John is referring to Jesus as the true God can be demonstrated from the fact that it is the Son who is declared to be the Life that gives everlasting life to all who believe in him:
“For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish, but have eternal life. For God did not send His Son into the world to condemn the world, but that the world through Him might be saved. He who believes in Him is not condemned. But he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God.” John 3:16-18
“He who comes from above is above all. He who is of the earth is earthly and speaks of the earth. He who comes from heaven is above all. He bears witness of what He has seen and heard, yet no one receives His testimony. He who has received His testimony has certified that God is true. For He whom God has sent speaks the words of God, for God gives the Spirit without measure to Him. The Father loves the Son, and has placed all things into His hand. He who believes in the Son has eternal life. He who does not believe the Son shall not see life, but the wrath of God remains on him.” John 3:31-36
“For as the Father raises the dead and gives them life, even so the Son gives life to whom He will… Truly, truly I say to you, the hour is coming, and is now here, when the dead will hear the voice of the Son of God, and those who hear will live. For as the Father has life in Himself, so He has given to the Son to have life in Himself… Do not marvel at this. For the hour is coming in which all who are in the graves will hear His voice and come out—those who have done good to the resurrection of life, and those who have done evil to the resurrection of judgment.” John 5:21, 25-26, 28-29
“‘All whom the Father gives Me will come to Me, and he who comes to Me I will never cast out. For I came down from heaven, not to do My own will, but the will of Him who sent Me. This is the will of the Father who has sent Me, that of all whom He has given Me, I should lose nothing, but should raise it up at the last day. This is the will of Him who sent Me, that everyone who sees the Son and believes in Him may have eternal life, and I will raise him up on the last day.’ The Jews then murmured about Him, because He said, ‘I am the bread which came down from heaven.’ They said, ‘Is this not Jesus, the son of Joseph, whose father and mother we know? How is it then that He says, “I have come down from heaven”?’ Jesus therefore answered them, ‘Do not murmur among yourselves. No one can come to Me unless the Father who has sent Me draws him. And I will raise him up on the last day… Whoever eats My flesh and drinks My blood has eternal life. And I will raise him up on the last day.’” John 6:37-44, 54
“‘My sheep hear My voice, and I know them, and they follow Me. I give them eternal life. They shall never perish, nor shall anyone snatch them from My hand. My Father, who has given them to Me, is greater than all. No one is able to snatch them from My Father’s hand. My Father and I are one.’ Again the Jews took up stones to stone Him. Jesus answered them, ‘I have shown you many good works from My Father. For which of those works do you stone Me?’ The Jews answered Him, ‘We are not stoning You for a good work, but for blasphemy, and because You, being a Man, claim to be God.’” John 10:27-33
“Jesus said to her, ‘I am the resurrection and the life. He who believes in Me, though he may die, yet shall he live. And whoever lives and believes in Me shall never die. Do you believe this?’” John 11:25-26
“Jesus said to him, ‘I am the way, the truth, and the life. No one comes to the Father except through Me.’” John 14:6
“If we receive the testimony of men, the testimony of God is greater; for this is the testimony of God which He has given concerning His Son. Whoever believes in the Son of God has this witness in himself. Whoever does not believe God has made Him out to be a liar, because he does not believe the testimony that God gave about His Son. And this is the testimony: that God has given us eternal life, and this life is in His Son. Whoever has the Son has life, and whoever does not have the Son of God does not have life. I have written these things to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God. This is the confidence that we have in Him, that if we ask anything according to His will, He hears us. So if we know that He hears whatever we ask, we know that we have whatever we asked of Him.” 1 John 5:9-15
There is additional corroboration from the epistle of John itself, since it begins by bearing witness to Christ being that very eternal life that was with the Father and who manifested himself to John and the other disciples:
“That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have touched, concerning the Word of life—the life was revealed, and we have seen it and testify to it, and announce to you the eternal life, which was with the Father and was revealed to us—we declare to you that which we have seen and heard, that you also may have fellowship with us. And our fellowship is with the Father and with His Son Jesus Christ.” 1 John 1:1-3
Hence, John ends his letter the way he started it, namely, by testifying that the Son is himself the very eternal life who has come to grant everlasting life to all who would believe in him. As the following scholars explain:
ba. 1 John 5:20 sn The pronoun This one (οὗτος, houtos) refers to a person, but it is far from clear whether it should be understood as a reference (1) to God the Father or (2) to Jesus Christ. R. E. Brown (Epistles of John [AB], 625) comments, “I John, which began with an example of stunning grammatical obscurity in the prologue, continues to the end to offer us examples of unclear grammar.” The nearest previous antecedent is Jesus Christ, immediately preceding, but on some occasions when this has been true the pronoun still refers to God (see 1 John 2:3). The first predicate which follows This one in 5:20, the true God, is a description of God the Father used by Jesus in John 17:3, and was used in the preceding clause of the present verse to refer to God the Father (him who is true). Yet the second predicate of This one in 5:20, eternal life, appears to refer to Jesus, because although the Father possesses “life” (John 5:26; 6:57) just as Jesus does (John 1:4; 6:57, 1 John 5:11), “life” is never predicated of the Father elsewhere, while it is predicated of Jesus in John 11:25 and 14:6 (a self-predication by Jesus). If This one in 5:20 is understood as referring to Jesus, it forms an inclusion with the prologue, which introduced the reader to “the eternal life which was with the Father and was manifested to us.” Thus it appears best to understand the pronoun This one in 5:20 as a reference to Jesus Christ. The christological affirmation which results is striking, but certainly not beyond the capabilities of the author (see John 1:1 and 20:28): This One [Jesus Christ] is the true God and eternal life. See also D. B. Wallace, Granville Sharp’s Canon and Its Kin: Semantics and Significance, Studies in Biblical Greek 14, ed. D. A. Carson (Bern/New York: Peter Lang, 2009), 273-77. (NET Bible https://www.biblegateway.com/passage/?search=1+john+5&version=NET; bold emphasis mine)
Therefore, Jesus is indeed the true God and eternal life, not in distinction from the Father but in perfect inseparable union with the One whom he proclaimed to be the only true God. This is why everlasting life is dependent on knowing the Son in the same sense and to the same extent that one is to know the Father, since you cannot truly know the Father without knowing the Son who came to perfectly reveal him.
Biblical references are taken from the Modern English Version (MEV) of the Holy Bible.
FURTHER READING
JESUS CHRIST: TRUE GOD FROM TRUE GOD
Answering Islam Blog 
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