1 Jn 5:20 -
"We are in him that is true [alethinos], even in his Son, Jesus Christ. This [outos] is the true [alethinos] God, and eternal life." - KJV.
Some trinitarians actually insist that the word "this" (outos) here refers to Jesus. In other words, "[Jesus Christ] is the true God and eternal life." For example, Robert M. Bowman in his Jehovah's Witnesses, Jesus Christ, and the Gospel of John states that at 1 Jn 5:20 Jesus is called `the true God and eternal life' "indisputably identifying Christ as the Almighty God of the Old Testament." - p. 41, Baker Book House, 1991 printing. I understand why some trinitarians are so desperate in their search for non-existent scriptural "evidence" that they have to make it up, but this is incredibly poor!
It is obvious that grammatically the word "this" (outos) could be referring to either the Father or Jesus in this particular scripture (see the footnote for 1 John 5:20 in the very trinitarian NIV Study Bible). But the fact that the true God (or "the true One") has just been identified as the Father of Jesus (1 Jn 5:20, TEV and GNB) makes it highly probable that "this is the true God" refers to the Father, not Jesus. The highly trinitarian NT scholar Murray J. Harris sums up his 13-page analysis of this scripture as follows:
"Although it is certainly possible that outos refers back to Jesus Christ, several converging lines of evidence point to `the true one,' God the Father, as the probable antecedent. This position, outos = God [Father], is held by many commentators, authors of general studies, and significantly, by those grammarians who express an opinion on the matter." - p. 253, Jesus as God, Baker Book House, 1992.
Notice how this trinitarian scholar actually admits that the probability is that the Father (not Jesus) is being called the true God here. He even tells us (and cites examples in his footnotes) that New Testament grammarians and commentators (most of them trinitarian, of course) agree!
So this single "proof" that the "true God" is a title for anyone other than the Father alone is not proof at all. The grammar alone merely makes it a possibility. The immediate context makes it highly improbable since (as in all other uses of the term) the true God (or the true one) was just identified as the Father ("We are in the one who is true as we are in his Son, Jesus Christ. He is the true God and this is eternal life." - NJB; and "We know that the Son of God has come and has given us understanding, so that we know the true God. We live in union with the true God - in union with his Son Jesus Christ. This is the true God, and this is eternal life." - TEV.).
So the immediate context alone makes it probable that the true God is the Father in this scripture also. As we have seen, if we include the context of all the uses of the `true God,' it is certain that He is the Father alone (whose personal name is Jehovah - Ps. 83:18, Ex. 3:15).
To clinch John's intended meaning at 1 John 5:20, let's look at his only other use of the term: John 17:1, 3, where, again (as in 1 Jn 5:20), he mentions Father, Son, and eternal life. Here the Father alone is not only very clearly identified as the only true God, but Jesus Christ is pointedly and specifically excluded from that identification ("AND Jesus Christ whom YOU [the only true God] have sent").
For more, see:
"The Only True God"