“Know” is the Greek 2nd person, plural, subjunctive mood of ειδητε from the second perfect tense, of which the indicative οιδα is commonly translated in classical and Koine Greek as “I know” or “you know.” All the other Greek moods take their conjugations from the irregular verb οιδα, which means “to see,” and as such, οιδα and ειδω are used interchangeably in the New Testament. John does so in many cases (cf. 1Jn 2:20, 21, 29). These words appear over 600 times in the New Testament. In each case, the kind of “knowing” is conditioned by the entity doing the knowing and the thing known. John frequently speaks of factual knowledge about the Christian faith that can be known absolutely (e.g., 1Jn 3:15 — “we know that no murderer has eternal life”; 1Jn 5:20 — “we know also that the Son of God has come”), yet knowledge that is also conditioned on how God wishes to reveal it to us (e.g., 1Jn 5:15 — “And if we know that he hears us, whatever we ask, we know we have what we have asked of him.”). In any case, except for one instance, John invariably uses “know” in the plural verb form, denoting that it is the community of believers who possess the factual knowledge of the aspects of the Christian faith, not the individual possessing factual certainty of his own salvation without contingencies (cf. 1Jn 2:20, 21, 29; 3:2, 5, 14, 15; 5:13, 15, 16, 18, 19, 20; 3Jn 12, 14). John’s use of “know” in the singular appears in 1Jn 2:11. Here, however, he is speaking of an individual in sin who does not know where he is going. (Robert A. Sungenis, Not By Faith Alone: The Biblical Evidence for the Catholic Doctrine of Justification [2d ed.; State Line, Pa.: Catholic Apologetics International Publishing, Inc., 2009], 456 n. 631 [print ed.])