The Living Bible by Kenneth Taylor here reads: "Jesus Christ his Son, who is the only true God; and he is life eternal."

If the above is correct then we would have to accept that Jesus Christ is "the only true God." As the Bible clearly states that there is only one true God then this might mean that John was saying that the Son, aswell as the Father, were this "God". Hence, this passage could be used to support the teaching that God is a being who is also three persons(the other person being the 'Holy Spirit). The 'Trinity'.

The Greek here reads:

"oidamen de hoti ho huios tou theou hekei kai dedwken heuin dianoian hiva ginwskomen ton alethinon: kai esmen en tw alethinos en tw huiw autou Iesou Christw. Houtos estin ho alethinos theos kai zwe aiwnios."

Literally this is "We-have-known but that the Son of-the God is-come, and he-has-given to-us mental-perception in-order-that we-are-knowing the true(one); and we-are in the true(one), in the Son of-him to-Jesus Christ. This(one) is the true God and life everlasting."- Kingdom Interlinear Translation, WTB&TS, 1986 edition.

From the above it is quite clear that John is stressing "that"('hoti') the Son of God has "come" ('hekei'). And why has the Son come? It was to "give us mental-perception"('diaonian'). But about whom should we gain this "mental-perception? John writes "in-order-that we-are-knowing *the true one*('alethinon'). Did not the Son come to explain the Father who is "ton monon alhthinon theou," that is, "the only true God" ?- John 17:3. Is not John focussing on the Father in verse 18,19 and this sets up what he writes in v.20? So the subject of "houtos," "This(one)," is manifestly "the true God and life everlasting." Hence, although the closest possible antecedent to "houtos," "This one," is Jesus Christ, it is the "true one," that is- God the Father.

"houtos: as a climax to vv.18-20 the ref[erence] is almost certainly to God the real, the true, opp[osite of] paganism(v.21.)"- "A Grammatical Analysis of the Greek New Testament, Zerwick/Grosvenor, Rome Biblical Institue, 1981.

"[1 John 5:]20f. Christ has revealed the one true God, the source of eternal life(cf. 5:12; Jn 17:3, 20:31). 'This is the true God' does not refer to Jesus as Stauffer thinks(Theology of the NT.(English translation 1955), 114)." G. Johnston, Peake's Commentary on the Bible, Thomas Nelson and Sons, reprint of 1964.

"Conclusion: Although it is certainly possible that houtos["this one"] refers back to Jesus Christ, several converging lines of evidence points to "the true one," God the Father, as the probable antecedent. This position, houtos = God, is held by many commentators, authors of general studies, and significantly, by those grammarians who express an opinion on the matter."-M. Harris, "Jesus as God, The New Testament Use of Theos in Reference to Jesus," Baker Book House, Grand Rapids, Michigan, 1992, p.253.

"..it is more likely that the word "this" has a wider and vaguer reference. The writer is gathering together in his mind all that he has been saying about God- how He is light and love, how He is revealed as the Father through his Son Jesus Christ, and "this", he adds, "is the real God"...For illustration of this we need only recall John 17:3." C.H.Dodd, Moffatt New Testament Commentary.

"[1 John]5.20-21. Knowing the true God;... The Greek of 5:20 has only the true (one) and reads literally: we know that the Son of God has come and has given us understanding 'so that we know the true(one) and we are in the true(one)', in his Son Jesus Christ. 'This (one) is the true God and eternal life.' It is clear from this that 'the true (one)' is God throughout. Christ is his Son. In the final sentence this (one) most naturally refers still to God, not to Christ, as some have suggested. It is not unknown for Christ to be given God's name(Phil. 2:9-11) or even to be called 'God' (Heb. 1:8-9; John 1:1), but that would run contrary to the theme here, which is contrasting true and false understandings of God for which Christ's revelation is the criterion.
"5:20 reminds us of Jesus' prayer according to John 17:3: 'This is eternal life: to know you the only true God and Jesus Christ whom you have sent...."- William Loader, The Johannine Epistles, Epworth Commentaries, 1992, p.79.(This commentary uses the Revised English Bible (1989) for it's quotations.)