The verse refers to the love which Christ has shown, and which Christians should show in imitation of Christ.

By this we know love, that he laid down his life for us: the introductory by this (compare 2.3) points forward to the that clause, which indicates by what means we know love. The pronouns he and his refer to Christ. The clause may better be transposed; for example, ‘the fact that he/Christ laid down his life for us causes us to know love,’ or ‘Christ laid down his life for us. Thus he showed us what love is.’ Where the noun love has to be rendered by a verb, one may say, for example, ‘Christ has caused us to know (or has shown us) how to love (or what it is really to love) when he laid down his life for us.’ If it is obligatory to mention a subject and a goal of the verb “to love,” it is probably best to say ‘… how we should love our brothers.’

The verb “to know” is used here with a nonpersonal goal; similarly in 3.20 (“God … knows everything”), 4.16 (“we know … the love God has for us”), and 2 John 1 (“all who know the truth”). In this construction the meaning of the verb closely resembles that of “to know” with a personal goal (compare 2.3), namely, to have intimate knowledge of something, realizing what it is like, and what are its practical implications. Some languages have the same expression in both cases, while in others one has to use different renderings.

He laid down his life for us: in this expression life is viewed as a cloak which one can lay down or take off. The aorist tense is used to show that the reference is to a specific event in history, namely, Jesus’ death. Some other renderings used are ‘he gave/offered his life for us,’ ‘he was willing (or ready) to die for us,’ (compare Good News Translation‘s renderings of the same expression in John 10.11, 15, 13.37).

He, literally “that one,” refers to Christ; see comments on 2.6. For life the Greek uses psuchē, for which see comments on “life” in 1.1. The preposition for may be rendered as ‘on behalf of,’ ‘for the sake of,’ ‘in order to help (or save).’

Since the following sentence draws a conclusion, and can be rendered as ‘so,’ ‘consequently.’

We ought: the pronoun is emphatic; it contrasts with “he” in the preceding sentence. The present tense of the Greek verb indicates duration. For the meaning and some renderings of the verb, see comments on 2.6.

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 .