The Text:
Joh 17:3 And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
Muslim Reply to My Claim:
I’ve not misunderstood John 17:3, you have. Do we have to go to the dictionary to understand the meaning of “only”
really 
Let’s read the verse.
[John 17:3]
Now this is eternal life: that they know you, the only true God, and Jesus Christ, whom you have sent.
That means no one else can be true God
Father alone is the only true God. So problematic was this passage for later Trinitarians in the 3rd and 4th century that they changed it when they were commenting on this very passage. One of the examples is Saint Augustine (354 – 430 C.E.) Bishop and Theologian, when he was commenting on this particular verse of the Gospel of John, he actually changed the text to substantiate the doctrine of the trinity.
My Response:
It is very interesting to me that you should suppose that I have misread John 17:3!
John 17:3 teaches that we can only have eternal life if we continue to know both God the Father AND continue to know Jesus Christ His Son.
That Jesus is God’s Son is stated at the beginning of this prayer when Jesus prayed:
John 17:1 These words spake Jesus, and lifted up his eyes to heaven, and said, Father, the hour is come; glorify thy Son, that thy Son also may glorify thee: (KJV)
Jesus is praying to His Father. Jesus is the Father’s Son. The Father is God. Therefore, Jesus is God’s Son.
In John 17:2 Jesus prays and asserts that God the Father has given to Jesus the exclusive authority over all mankind to give eternal life to all those the Father has given to Jesus:
Joh 17:2 just as you have given Him authority over all mankind to give eternal life to all whom you have given Him. (Williams NT)
That this authority is exclusive and is given to Jesus alone is stated by Jesus Himself when Jesus said:
John 14:6 Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.
Jesus spoke of this authority, given exclusively to Him by the Father, in Matthew 11:27,
Matthew 11:27 All things were handed over to Me by My Father, and no one fully knows the Son except the Father. Nor does anyone fully know the Father except the Son, and to whom the Son may will to reveal Him. (Lavenders’ NT)
John 17:3 And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent. (KJV)
John 17:3 Now eternal life means knowing you as the only true God and knowing Jesus your messenger as Christ. (Williams NT)
Jesus identifies the Father as the only true God. The Father, clearly, is the only true God in opposition to or in contrast with all other false gods.
Jesus is not speaking of His Father in opposition to Himself or in contrast with Himself in a manner that denies His own Deity. Anyone who has carefully read the entire Gospel of John learns that from beginning (John 1:1, “and the word was God”) to its climax (John 20:28, “my Lord and my God”) that the Gospel of John was written for a specific purpose:
Joh 20:31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name. (KJV)
Joh 20:31 But these have been recorded, in order that you may believe that Jesus is the Christ, the Son of God, and that through believing you may have life, as bearers of His name. (Williams NT)
Jesus then continues His prayer in John 17,
Joh 17:4 I have glorified you down here upon the earth by completing the work which you have given me to do. (Williams NT)
Consider carefully the implicit claim Jesus makes in His prayer (John 17:5) that He, Jesus, was with the Father for all eternity past in heaven as the Son of God:
Joh 17:5 And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was. (KJV)
Joh 17:5 So now, Father, glorify me up there in your presence just as you did before the world existed. (Williams NT)
Joh 17:5 Father! Give me glory in your presence now, the same glory I had with you before the world was made. (GNB, Good News Bible)
Joh 17:5 Now, Father, give me back the glory that I had with you before the world was created. (CEV, Contemporary English Version)
Both Muslims and Jehovah’s Witnesses fail to understand that there are three Persons in the One Godhead (Matthew 28:19, “in the name,” singular). In the Bible the Father is spoken of as God (John 6:27). Jesus is called God (John 20:28). The Holy Spirit is called God (Acts 5:3, 4).
In Jesus the fulness of the Godhead dwelt bodily:
Colossians 2:9 For in him dwelleth all the fulness of the Godhead bodily. (KJV)