This verse draws a conclusion; hence, for example, “I write you, then, …” (Good News Translation).

I write to you: for the pronoun see 2.1; for the verb see 1.4. The tense is the aorist, as in verses 13c and 14; see comments on I am writing in 2.12. In the three following clauses the verb “to know” occurs only twice in the Greek. The third occurrence in Revised Standard Version is an addition, for which see below.

Because … because … that … The Greek uses the same conjunction three times, and it can mean “because” or “that.” Three interpretations are given: (1) three times “because,” dependent on “I write” (New English Bible, and others); (2) three times “that,” dependent on “I write” again; and (3) twice “because” or “that,” dependent on “I write,” once “that,” dependent on the second “you know.” In case (3) the verb is supposed to have two direct objects, the first a pronoun, the second a clause: “but because you know it (that is, the truth), and that no lie is of the truth.” Such a construction, though harsh and rather unusual, is not impossible, and it makes good sense. It is represented, among others, by Revised Standard Version and Good News Translation, both of which add a third “know” in order to ease over the harshness of the construction.

The present authors think (1) unlikely, and (3) quite defensible, but they have a slight preference for (2). According to that interpretation the writer wants to assure his readers that it is they, not his opponents, who really know the truth. To express himself as strongly as possible, he uses a pair of opposite clauses that reinforce each other, stating the same fact first negatively, then positively. This is a characteristic feature of his style; compare similar negative-positive or positive-negative pairs in 1.6, 8; 2.4, 7, 16, 23, 27b; 4.18; 3 John 11. And for further emphasis he adds the third clause, which states that there is a basic and essential difference between truth and untruth.

You do not know the truth, or ‘you are not aware of what is (really) true.’ For the truth see comments on 1.6, but the reference here is to facts about Christ. This is shown by the next verse: the truth John’s readers know is the fact that Jesus is the Christ, as revealed in the gospel.

No lie is of the truth: for “to be of” see comments on 2.16. The clause aims directly at the teaching of John’s opponents. Just as they themselves are not “of us” (verse 19), so their words are lies and cannot “be of the truth,” or ‘spring from the truth,’ ‘have the quality of truth’; or, with further shifts, ‘lies and truth cannot go together,’ ‘one who lies cannot have anything to do with truth.’

Accordingly the topic of this last clause of verse 21 is lie. Therefore an otherwise acceptable transposition, ‘truth cannot produce a lie,’ may be less advisable. For lie, or ‘false/untrue word,’ ‘what is false/untrue,’ see comments on the verb in 1.6.

The truth has another shade of meaning than in the first part of this verse. Now it refers to God’s real being (see comments on 1.8); its function comes close to that of a name of God. Therefore it is sometimes to be rendered ‘the true One,’ ‘the One who is true.’

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 .