The transition from the preceding discourse on the prophets to the discourse on false teachers is this. In the people of God the prophets spoke the word of God according to the will of God. Therefore their word, or teaching, was indisputably true as the word of God. Yet alongside them, these holy men, there were also false prophets. God had not sent them, and their teaching, or word, was false. So also among Christians there will appear, and already have appeared, false teachers. Their teaching is not true, and is not from God or from Christ.
"False prophets" (cf. Mt 7:15; Mt 24:11 and note) were men who presented themselves as true prophets, extraordinary messengers of God, though they were not such. Their distinguishing traits are these. They assure others, without proof, that God has sent them to announce His will to men, though God has not sent them (Jer 23:21; Jer 27:15). They deceive the people, falsely promising peace and security in God's name, when there is no peace or security (Ezek 13:3 and following). They partly deceive the people by confidence in external temple worship. Partly they grossly lead them to impiety, while they themselves do not believe God and His law (Jer 7:4). They hated and insulted the true prophets, and incited the people to murder them (Jer 26:8, 11; cf. Mt 23:37).
False prophets appeared often among the Hebrew people. Moses already mentions them (cf. Deut 13:1-5; Deut 18:20 and following). They multiplied especially around the time of the Babylonian captivity. The Lord Himself foretold that false prophets would appear in His Church (Mt 24:11 and note). Such persons later did indeed appear (Acts 20:30; 1 Jn 4:1).
The Apostle, however, does not call such persons false prophets by that precise name, but false teachers. Yet this has the same meaning. False prophets are false teachers, and false teachers are false prophets. The first term characterizes the Old Testament viewpoint more; the latter, the New Testament viewpoint. "By the name of prophecy, commonly applied to both prophets and false teachers, they warned the faithful not to heed false prophets" (Theophylact).
The Apostle Paul also characteristically calls them speakers of lies (1 Tim 4:2). These are people who do not teach what the true word of God, or Holy Scripture, teaches. Instead they corrupt or distort it, reinterpreting it and inserting into it another sense and meaning (2 Cor 2:17).
Having rejected and distorted the true apostolic teaching, they will introduce destructive heresies among you, or divisions on one or another point of doctrine (cf. Acts 5:17). Such divisions are called destructive because, if they concern the fundamental points of Christian doctrine, they lead to destruction and ruin for eternal life. They reject or distort one or another truth of doctrine. They are destructive also because they produce ruinous disorders, quarrels, and strife within the community. They are destructive both individually for each person and communally for all (cf. 2 Pet 2:2-3).
"Denying": that is, repudiating and denying (cf. Mt 10:33). Such denial or rejection takes place in the heart and mind through unbelief. On the lips it happens through blasphemy, or through insincere or incorrect confession. In deed it happens through an unchristian way of life and conduct. The opposite of this act of denial is confession with heart, lips, and deed, or with one's life (cf. Rom 10:10; 1 Cor 12:3; 2 Tim 1:12; 1 Tim 6:12).
Here the meaning is not such a complete and total denial or rejection of Christ that His messianic dignity is denied, and He is not acknowledged as true God. In that case such false teachers would not be Christians. They would be outside the Church. Yet, according to the context, they were considered and considered themselves to be in communion with the Church.
Here denial means a reinterpretation or distortion of the dogma of redemption as the Church confesses it. Perhaps, and probably, they considered themselves Christians. Yet by their false teaching they denied and rejected Christ, Who had redeemed them and the whole world. Because of ancestral sin the whole world was guilty before God and subject to eternal condemnation. But by His death and blood Christ offered to God a propitiatory sacrifice for all mankind. He redeemed mankind from sin, curse, and death, and so became the Redeemer of all (see the note on Mt 20:28).
"Swift destruction": that is, temporal and eternal death. This destruction will be swift. The word is used either in the sense of something soon to come (2 Pet 1:14), or in the sense of something that strikes suddenly and at once (Mt 24:29). According to the course of the discourse, the first meaning should be preferred to the latter (cf. 2 Pet 2:3).