The proposition stated in verse 3 starts from the conviction that a man’s visible behavior and his invisible relation to God are so closely parallel that one can draw conclusions from the one concerning the other. Accordingly, from the fact that a man keeps God’s commandments, one can infer that he knows God; the former is the proof of the latter.
And does not have connective or transitional force here but serves to emphasize the subsequent by this. In several languages it is better left untranslated.
The prepositional phrase by this…, if … points forward to the dependent clause and is explained by it. The construction is chosen for reasons of emphasis. It serves to focus attention on the keeping of the commandments.
It is often preferable in translation to change the sentence structure. Using a more common sentence type one may say ‘if we keep God’s commands, then we can be sure that we know him’ (compare Good News Translation), or somewhat more expressively, “it is only when we obey God’s laws that we can be quite sure that we really know him” (Phillips). Other possibly useful restructurings are ‘if we keep God’s commands, we have the proof that we know him’ or “here is the test by which we can make sure that we know him: do we keep his commands?” (New English Bible).
Constructions with by this (or “in this,” in 4.9, 17, rendering the same Greek phrase) in the main clause pointing forward to an explanatory dependent clause occur a few times in the Gospel of John and are rather common in the present Letter. The dependent clause may be introduced by if, as is the case here and in John 13.35. This seems to indicate that the statement is to be viewed as a reference to assumed fact. A similar case, but using the conjunction “when/as often as” (Greek hotan) occurs in 5.2 (if there the alternative interpretation is followed). Elsewhere in this Letter the clause is introduced by “that” (3.16; 4.9-10, 13, 17), the statement being viewed as a reference to actual fact. In some cases no connective is used (4.2; also, probably, 3.10), but then again the clause seems to refer to fact.
The clause by this we may be sure serves to call attention to the subsequent statement of an important Christian truth, either in general or specifically applying to the situation of the readers of this Letter. The same or similar expressions, pointing to subsequent statements of the same kind, are found in 3.19, 24; 4.2, 13. In 2.5 and 5.2 it is not certain whether the expression points forward or backward; see the verses in question.
To be sure (or “to know”) refers to being aware of truth, or to discerning between what is true and what is not true (in one language expressed negatively, ‘not mistaking-the-one-for-the other’). Often “to be sure by” is better rendered ‘to be made sure by,’ ‘to be shown by.’ With a further shift this may lead here to a rendering like ‘this makes us sure,’ ‘this shows us,’ ‘this proves to us.’
We know him: in this and the next verse him may theoretically refer either to God or to Christ. The former is preferable because it is unlikely that this pronoun would have another reference that the possessive pronoun has in “his (that is, God’s) commandments”; see below. But one should bear in mind that for John there is no sharp distinction between God and Christ in contexts like this; to know or obey the one means to know or obey the other.
† The verb “to know” means in this passage “to have become (or to be) intimately acquainted with,” namely, with a person’s intentions and character. To bring this out some versions have here ‘to know how God is,’ ‘to know God, what he is like.’ The expression implies fellowship and communion with God. But for John, knowing God does not mean a mystic union with God, detaching oneself from earthly things, as it probably meant for his opponents. It has ethical implications of obedience towards God’s commandments (compare also 4.7-8). What he says here about the knowledge of God should be viewed in the light of such eschatological Old Testament passages as Jer 31.31-34. Other references to the Christians’ knowledge of God occur in 2.4, 13-14; 3.1, 6; 4.6-8; 5.20.
We keep his commandments, an expression that is characteristic of the Johannine writings. By keeping God’s commandments one shows one’s love for him (1 John 5.3), abides in him (3.24), and can be confident that he will hear one’s prayer (3.22). Conversely, one who does not keep God’s commandments is a stranger to the truth of God (2.4). The phrase is virtually synonymous with the expression “to keep his word” (compare verse 5).
† “To keep” may in this context be rendered by ‘to observe,’ ‘to obey,’ ‘to listen to.’ Some verbs used basically mean ‘to guard,’ ‘to complete/fulfill,’ ‘to hold-in-remembrance,’ ‘to confirm/agree,’ ‘to do-according-to.’ Other occurrences of the verb in this meaning are in 2.5; 3.22, 24; 5.3, and compare comments on “disobeys” in 2.4.
† With perhaps one exception (compare 3.23) the noun commandments is always used in John’s Letters to refer to what God orders, or tells, people to do; hence the possessive pronoun in the present verse should be interpreted as referring to God, not to Christ. The plural (2.3-4; 3.22, 24; 5.2-3; 2 John 6) is used to indicate that the reference is to deeds which give concrete form to the one, great commandment of love (in the singular, see 2.7-8; 3.23; 4.21; 2 John 5-6, and compare “word” in 1 John 2.5). The noun sometimes has been rendered by ‘rule,’ ‘what has been laid down,’ ‘what one should follow,’ ‘what one is-caused-to-follow.’
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 .