This is one of the 10 famous possible “Jesus as God” verses which are much discussed by Trinitarians and non-Trinitarians examining in detail the fine points of grammar and variations in the Greek manuscripts.
Houtos – ‘this’ or ‘he’, and if ‘He’, then who?
The argument here hinges on a point of Greek grammar; whether the word “This” (houtos) refers to the previous statement “in him who is true” – about God himself, – or to the last words “even in his Son Jesus Christ”.
The KJV leaves it as a statement “this”:
19. And we know that we are of God, and the whole world lieth in wickedness. 20. And we know that the Son of God is come, and hath given us an understanding, that we may know him that is true, and we are in him that is true, even in his Son Jesus Christ. This is the true God, and eternal life. (KJV)
Most modern versions refer the ”houtos” as “He” – although modern versions also remain neutral in that they do not indicate whether “He” refers to God or Jesus:
20 And we know that the Son of God has come and has given us understanding so that we may know him who is true;[a] and we are in him who is true, in his Son Jesus Christ. He is the true God and eternal life. (NRSV)
The [a] footnote in the NRSV reads : Other ancient authorities read “know the true God”. This indicates the main textual variant in this verse.
Two interesting exceptions to the modern reading of “He (God or Jesus) is the true God” are the Contemporary English Version and Good News Translation:
We know that Jesus Christ the Son of God has come and has shown us the true God. And because of Jesus, we now belong to the true God who gives eternal life. (CEV)
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. (GNT)
Conclusion
It is not possible to conclude with certainty either way what “this” or which of two possible “He” 1 John 5:20 refers to – so this verse is unlikely to be a deciding verse for anyone. The context however is about God being true, and the verse already states that God is “the true one”, or “the true God” according to different manuscripts.
Further reading:
One God or a Trinity? James and Deb Flint
Did Jesus exist before his birth? A.E. Walker and John Shemeld.
See also other answers on BibleQ:
Does the Bible call Jesus ‘God’? (10 verses which some say do) This answer links to 10 other answers looking at 10 verses.