"The Greek of [1 Jn] 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"
(The Johannine Epistles. London: Epworth Press, 1992.
While I disagree with Loader's take on Jn. 1:1 and Heb. 1:8-9, what he writes about 1 Jn. 5:20 seems to represent the context faithfully and it dovetails with what others such as M. J. Harris., M. Zewick and J. Moulton have pointed out about this particular Christologically-significant Johannine passage.