The first part of this verse quotes a proposition of the false teachers and then characterizes their behavior. The next part proceeds to refute that proposition and to blame that behavior.
If any one says resembles the clauses that serve to introduce the propositions of the false teachers in 1.5–2.11; compare comments on “if we say” in 1.6 and on “he who says” in 2.4. The characterization given of those men is in accordance with what is said in Part One; see especially 1.6; 2.4 and 9.
Hates his brother: the verb is in the present tense and expresses duration or habitual behavior. For “to hate” (paralleling “not to love” in verse 20b) and for brother (here and in verse 21), see comments on 2.9.
For he is a liar, compare comments on 1.10 and on “we lie” in 1.6.
Whereas verse 12 stated that the love God shows man is made perfect in men’s love for each other, verse 20b speaks of the absence of brotherly love and what its absence implies. The statement is based on the common experience that it is easier to love a person whom one sees than a person whom one does not see, and that this is especially so in the case of one’s brother, that is, a person who is like oneself. Starting from this John argues that one who does not do the easier thing, namely, loving one’s visible brother, will surely not be able to do the more difficult thing, loving the invisible God. Consequently lack of brotherly love shows that one does not love God.
If the sentence structure is felt to be too complex, one may say something like ‘He sees his brother, yet he doesn’t love him. He doesn’t see God, consequently he cannot love him/God,’ or ‘He does not love his brother whose face he sees. How can he love God? For his/God’s face he does not see.’
He who does not love his brother whom he has seen: the last verb is in the perfect tense, indicating that he saw and still sees. Receptor languages, however, often require a present tense.
Cannot or, according to another reading of the Greek, “how can he”: semantically the two readings do not differ, since the second one contains a rhetorical question anticipating a negative answer. The subsequent clause contains a climax which may have to be made explicit; for example, ‘certainly he cannot,’ ‘even less can he.’
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 .