If: the corresponding Greek conjunction followed by the subjunctive of the aorist may have conditional meaning, “in the case that,” or temporal meaning, “when,” “at the moment that.” Both interpretations are acceptable in this context.

Sees his brother committing … sin: the verb “to see” with the direct personal object and following participle refers to seeing the person in a certain action or state of being. The clause may have to be restructured; for example, ‘sees his brother who (or while he) is committing … sin.’ For brother see comments on 2.9.

Committing what is not a mortal sin is in the Greek literally “sinning a sin not to death.” Some other ways to render this construction are ‘committing a sin that is not to death,’ ‘doing a sin, but not a sin to death,’ ‘sinning, but not sinning to death.’ For “sin” and “to sin” see comments on 1.7 and 10.

A mortal sin, or ‘a sin that is deadly (or causes-to-die, or leads to death, or causes-to-come death).’ The reference is to sins that cut off the sinner from the life given him in Christ, and so cause him to fall into spiritual death. The sin John had in view may have been hate and lovelessness; compare “whoever does not love is still in death” (3.14, Good News Bible); or it may have been denial of Christ; compare “whoever does not have the Son of God does not have life” (5.12, Good News Bible).

The reference to death must be taken metaphorically here. In rendering the phrase it may be better to shift from metaphor to simile; for example, ‘sin which makes them with their head-hearts like dead people’; or it may be better to explain the metaphorical use; for example, ‘sin that will end in spiritual death,’ ‘sin that cuts (him) off from (eternal) life’; or again to use another metaphor, as in ‘for good taking the evil road.’

In some receptor languages ‘mortal sin’ is a well-known expression in the ethical terminology of the Church, but to use the word in this sense in John’s Letter would be an anachronism. Then one may have to coin a new phrase; for example, ‘sin that tends towards death’ (in one Philippine language, where ‘mortal sin’ would have legalistic overtones).

He will ask, that is, ‘he will ask/pray for him’: this clause is referring to a prayer of intercession. For this concept a specific term may exist (as in Arabic, for example), or a specific phrase such as ‘to plead as go-between,’ ‘to speak up on behalf of.’

And introduces what the result of this prayer will be.

God will give him life is in the Greek literally “he-will-give him life.” Revised Standard Version, New English Bible, Good News Translation, and others take the agent to be God, since only God can dispense eternal life. It is also possible, however, to take the one who asks as the subject. Then the clause is a case of indirect agency, for the one who asks is not able to give life himself but causes another, namely God, to do so. A comparable thought is expressed in James 5.20.

Both interpretations are acceptable, but the second one is, perhaps, slightly more attractive because of what one commentator has called “the continuity of the construction.” It is brought out in such a rendering as “he will pray for him and obtain life for him” (Translators’ Translation, compare also Moffatt, Goodspeed, Bible de Jérusalem). In some cases an exegetically neutral rendering may be possible; for example, ‘he should pray for him; that will give him life,’ ‘he should pray, and thus his brother will be caused to live (or will receive life).’

For those whose sin is not mortal: although this phrase is in the plural, it is in apposition to “him.” This construction allows John to generalize the preceding proposition and at the same time emphasize the qualification. It is often better to render the phrase as a new clause or sentence; for example, “—that is, when men are not guilty of deadly sin” (New English Bible), or ‘This applies to (or I mean) all those whose sin is not mortal.’

There is sin which is mortal: the restrictive qualification “what is not a mortal sin” (verse 16a) implied already the existence of mortal sin. This is here stated explicitly by the author.

Now, however, the question arises as to what a Christian must do when he sees a brother committing a mortal sin. John’s answer, careful but resolute, is given in what follows:

I do not say that one is to pray for that: according to the Greek word order, the phrase for that (literally “concerning that”) is to be taken with “to say” rather than with to pray, but this makes no important difference for the meaning of the sentence. So the translator is free to follow the construction that best fits the receptor language.

In for that the pronoun may have to be specified; for example, ‘concerning that (or such a) sin,’ ‘about mortal sin.’ If a verbal construction is preferable, one may say ‘about a person who is sinning in this way.’

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 .