In Greek New Testament the paragraph begins with [kai], a reading of doubtful authority. In the majority of translations this word is not represented, and rightly so.

By this is best taken as pointing backward (as in 4.6 and, according to one interpretation, in 2.5). Then the line of thought is: by the fact that we love in deed and in truth, we know that we are of the truth. To bring this out clearly Good News Translation has “this, then, is how we will know.”

We shall know is not in the present tense, as in verse 16, but in the future tense. This serves here to express that the knowledge depends upon the fulfillment of the condition laid down in verse 18. Consequently the future tense expresses what can or may happen rather than what will happen; hence ‘we can/may know.’

We are of the truth: some acceptable renderings are ‘we belong to God, who is true,’ ‘we have the quality of truth (or of the true One),’ compare comments on 2.16 and 21.

And (shall) reassure our hearts: the clause is dependent on by this and as such is parallel to we shall know that …. To make this clear one may have to say something like ‘and (by this also we) can/may reassure our hearts.’ The plural of hearts is required by English usage because the subject is in the plural. It has distributive force, for which some other languages prefer a singular form (as does the Greek).

In “to reassure one’s heart,” the Greek verb can mean “to convince,” “to persuade,” “to win over,” but also (in Matt 28.14, for example) “to conciliate,” “to pacify,” “to satisfy,” “to set at ease or rest.” The last mentioned meaning fits the context here.

The syntactic structure of the phrase may have to be changed; for example, ‘the heart feels sure (or rests).’ Some equivalent idioms are ‘to be one-hearted (that is, at peace),’ ‘to be happy (of) heart,’ ‘the heart can breathe’ (that is, is relieved, as of a big problem).

Heart (in this and the next two verses) represents a Greek word that refers to the bodily heart, the seat of physical life, then, by extension, to the heart as the center and source of the whole inner life. In some receptor languages the corresponding term may literally mean ‘liver,’ ‘gall,’ or another part of the body; see comments on heart in verse 17. Since the word also stands for the self as a responsible person, the present phrase may also be rendered ‘to reassure oneself (or one’s conscience)’ (compare Phillips and Goodspeed).

Before him, or ‘in his/God’s sight,’ ‘when (we stand) in God’s presence’: the phrase is used in a metaphorical sense.

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 .