Jamieson, Fausset and Brown on 2 John 1:7

7. As love and truth go hand in hand (2Jo 3, 4), he feels it needful to give warning against teachers of untruth.

For--giving the reason why he dwelt on truth and on love, which manifests itself in keeping God's commandments (2Jo 6).

many--(1Jo 2:18; 4:1).

are entered--The oldest manuscripts read, "have gone forth," namely, from us.

confess not ... Jesus ... in the flesh--the token of Antichrist.

is come--Greek, "coming." He who denies Christ's coming in the flesh, denies the possibility of the incarnation; he who denies that he has come, denies its actuality. They denied the possibility of a Messiah's appearing, or coming, in the flesh [Neander]. I think the Greek present participle implies both the first and the second advent of Christ. He is often elsewhere called the Coming One (Greek), Mt 11:3; He 10:37. The denial of the reality of His manifestation in the flesh, at His first coming, and of His personal advent again, constitutes Antichrist. "The world turns away from God and Christ, busily intent upon its own husks; but to OPPOSE God and Christ is of the leaven of Satan" [Bengel].

This is a, &c.--Greek, "This (such a one as has been just described) is the deceiver and the Antichrist." The many who in a degree fulfil the character, are forerunners of the final personal Antichrist, who shall concentrate in himself all the features of previous Antichristian systems.