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 another use of the term: John 17:1, 3, where, again (as in 1 Jn 5:20), he mentions Father, Son, and eternal life.
At John 17:1, 3 Jesus prays to the Father: “Father, .... this is eternal life: that they may know you, the only true God, and Jesus Christ whom you have sent.” - New International Version (NIV). Here the Father alone is not only very clearly identified as the only true [alethinos] God, but Jesus Christ is again pointedly and specifically excluded from that identification (“AND Jesus Christ whom you [the only true God] have sent”)!
Notice how this respected trinitarian Bible has rendered John 17:1, 3 - “Father,....This is eternal life: to know thee who alone art truly God, and Jesus Christ whom thou hast sent.” - New English Bible (NEB).
So, the title “the true God” does not have to mean that there are no others who may be called “gods” or “a god” in a subordinate but righteous sense. It is, however, an exclusive title for God, the Most High, only true God, Jehovah. And clearly it refers exclusively to the Father! No one else is the God or the True God! (Compare Ps. 86:10; 2 Kings 19:19; Is. 37:16.)
"The Only True God" (Examining the Trinity)
1 John 5:20 "True God" (Defending the NWT)
1 John 5:20 (INDNWT)
If the Father is the "only true God" (John 17:3), does that mean that Jesus is a false god? (SFBT)
Translating "In" (DNWT)
The Bible’s Viewpoint - Is There Only One True God? (WBTS)
Is Jesus Christ God? WHO IS “the Only True God”? (WBTS)