And renders a Greek particle which may indicate a contrast here; the world is in the power of the evil one, but the believers are in the power of the true One. Or it may simply mark continuation. In the latter case the particle is sometimes best left untranslated; compare Good News Translation, New English Bible.
For the Son of God see comments on 3.8.
Has come is in the Greek a present tense form that has the force of the perfect tense. Similarly as the perfect of the next verb, it is used here to show that the event is still effective in the present.
Has given us understanding, to know him who is true: the noun understanding refers here to the ability to understand or perceive. What one can understand or perceive is defined in the subsequent words, to know him who is true. With some adjustments this may result in such renderings of the sentence as ‘he has given us power (or shown us the way) to know him…,’ ‘he has taught us how we can come to know him….’
In to know … the Greek verb is in the present tense, expressing continuation. For the meaning of the verb, see comments on 2.3.
Him who is true, or ‘the true one,’ refers to God. The phrase may have the connotation “the real one,” that is, the only genuine God (compare Isa 65.16; John 17.3; 1 Thes 1.9), or “the truthful/faithful one” (see above on 1.8, and compare Exo 34.6). The former connotation probably is prevalent here because of verse 21, which refers to “idols,” that is, to what is not real and genuine.
We are in him who is true states what is an actual fact, in contrast with the preceding to know him who is true, which states what may become a fact. Him who is true again refers to God. For “to be in” see comments on 1.8.
In his Son Jesus Christ: the phrase is dependent on we are and as such is parallel to “in him who is true.” By using this construction John suggests in a bried, clear way that the believers’ knowledge of and fellowship with God is possible only because of their knowledge of and fellowship with Christ, who is the mediator between God and men.
Several versions try to bring this out by using parallel sentences; for example, ‘we are one with him who is true, … we are one with his Son, Jesus Christ.’ Others make the connection explicit; compare, for example, ‘we live with the real God because we live with his Son, Jesus Christ.’
This is the true God: the demonstrative pronoun (in the Greek a masculine singular form) refers to Jesus; hence ‘this one (or he) is the true God,’ or, changing the sentence into a relative clause, ‘who is the true God.’
The true God is the predicate of this sentence. One may shift to the rendering ‘God, the true One.’
This passage is the only one in John’s Letters that equates Christ and God. In the Gospel, however, that equation is made a few times; compare John 1.1; 20.28; also 1.18 in Greek New Testament, compare Good News Translation, and footnotes to Revised Standard Version, New English Bible. It does not mean to say that Christ and God are one and the same being, but that in Christ we are dealing with God; hence renderings like ‘this one is what the true God is,’ ‘he means for us the true God.’
In the opinion of several commentators and translators, thus to equate Christ and God is not what the author intended to say here. Therefore some of them prefer to follow another interpretation which takes this as a reference to the preceding him who is true, referring to God. This seems to be a less acceptable solution, since in this context the statement then needlessly repeats the same idea.
Others suppose that the pronoun has a much wider and less clear reference here, namely, to all which the present Letter has said about God, his being revealed in his Son, and so forth. This interpretation, however, is not easily reconciled with the fact that the author has chosen the masculine form of the pronoun, not the neuter.
And eternal life is a second predicate with “this is.” As such it is syntactically parallel with “the true God.”
It is sometimes preferable to render this phrase as a full sentence, ‘and this one (or he) is eternal life.’ Then the relationship that exists between Christ and ‘eternal life’ may have to be described otherwise; for example, ‘he means (or gives) eternal life,’ ‘he causes people to live eternally,’ ‘through him we have true life (or live for ever).’ For eternal life see comments on 1.1-2.
Quoted with permission from Haas, C., de Jonge, M. and Swellengrebel, J.L. A Handbook on The First Letter of John. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .