They went out from us, but they were not of us. The meaning of this sentence turns on the understanding of from and of. Both are expressed by the same Greek preposition (“out-of”), which can indicate origin, but also membership of a group.
The first clause, they went out from us, is meant to draw attention to the fact that the antichrists had been members of the congregation, as well as to the fact that they left it. This is brought out in such renderings as, ‘these men went out from (or left) our company,’ ’ (it was) from among us (that) they went out.’—The verb “to go out from” is in the aorist, indicating that the reference is to a definite event in the past.
They were not of us. For “to be of” cp. v. 16. The clause serves to say that the antichrists (that is, the false teachers) have been members only in the outward appearance of things, not in the full sense of the word; hence, “these people really did not belong to our group” (TEV), ‘they were not our real companions,’ ‘their hearts were not fully the same as ours.’
Further rearrangement of the sentence pattern is sometimes idiomatically preferable. It may result in a rendering like, ‘they seemed to be (one) with us but now they have gone out.’
If they had been of us, they would have continued with us. The whole sentence is given in a form that shows it to be contrary to fact. The if-clause is the opposite of the preceding proposition.
The words they had been of us repeat what goes directly before, a repetition that is characteristic for John’s style. If idiom compels the translator to avoid such repetition, he may say, for example, ‘if that (really) had been so.’
The verb of the second clause, “to continue” (lit. “to remain”), is in the pluperfect in the Greek, which tense has the force of a combined aorist and imperfect. It serves here to state that the false teachers would have been with us in the past and would still be with us in the present. This is sometimes better expressed negatively, for example, ‘they would not have left us.’
But they went out, that, lit. “but in-order-that,” represents an ellipsis in the Greek. This ellipsis is to be filled out by repeating the verbal expression of the preceding clause (as RSV does), or by adding a more generic expression, for example,’ but this happened in order that.’
This construction occurs also in Mark 14:49 (the parallel Matt 26:56 is non-elliptical); John 1:8; 9:3; 13:18; 15:25. It often has the connotation of referring to something that is ordained by God. Therefore it can also be rendered by some phrase suggesting divine necessity. This may lead in the present sentence to a rendering like, ‘it had to become plain that they all are not of us.’
It might be plain is in the Greek lit. “they might-be-shown,” then, “they might become known”; or “they might show themselves.” The subject is the same as that of the preceding verbs in the verse, namely, the antichrists. Some receptor languages follow the non-personal construction of RSV, for example, ‘it might be shown,’ ‘it might become known,’ ‘it might become visible/clear.’ In others one can better shift to a rendering like, ‘we (inclusive) might see their situation clearly.’ The next clause indicates what that situation is.
They all are not of us. The Greek word order is, “not they-are all of us.” Rendered as in the RSV, the subject they refers to the antichrists, whereas “all” emphasizes they, and not negates the predicate are … of us. Another possibility is that not negates all, and all … not means “none”; hence, ‘none of them is of us,’ “none of them really belonged to our group” (TEV). Both interpretations are possible, and semantically they do not differ much, but the second one may be a better model of translation.
Quite a different interpretation is followed in versions that take all to be the subject and to refer to the congregation, cp. ‘not all are of us’ (NV), “not all in our company truly belong to it” (NEB). This interpretation would seem to be the less probable one for two reasons. (1) The shift of subject it presupposes would be rather unexpected in this verse. (2) To take not as restricting all (in the sense of, “not all, only some of them”) would require a different Greek word order, namely, “not all they-are of us” instead of “not they-are all of us” (cp. for example, 1 Cor 10:23)*