Burkitt:

BUT there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction.

Observe here, How the apostle foretells the coming of false teachers into the gospel church, as there had been false prophets in the Jewish church: no age of the church ever was or will be free of them; but the run of the last times is most likely to have most of these sour dregs. There shall be false teachers among you; false teachers then may find a scripture prophecy for their being in the church, but they will hardly find a scripture warrant for their being there. Observe, 2. The doctrines which they will teach: and they are damnable heresies. Where note. That Almighty God never intended a certain remedy against heresy, any more than he did against sin and vice; it is certain, that there is no certain and effectual remedy against either of them; God does what he sees best and fittest, not what we think to be so. Note also. That infallibility itself is no effectual remedy against heresy; the apostles were certainly infallible, and yet they could neither prevent nor extinguish heresy, which never more abounded than in the apostles’ times; St. Paul says, there must be heresies, 1 Cor. i. 19. St. Peter here says, that there shall be false teachers. Now, if there must be heresies and false teachers, either the church is not infallible, or infallibility is no effectual remedy against heresy. Observe, 3. That Christ is here called the Lord that bought these men who brought destruction upon themselves, Denying the Lord that bought them: because none should perish for want of a sufficient sacrifice for sin, Christ by his blood purchased for them pardon and life to be theirs, upon condition of believing acceptance. Observe lastly. As the seeds-men, false-teachers, and the seed they sow, damnable heresies, so the crop they shall reap, and that is, swift destruction: as damnable heresies are brought in privily, so the blasphemous heretic, the seducing heretic, the seditious heretic, brings upon himself swift destruction; sometimes temporal destruction in this world, certainly eternal, without repentance, in the next.

William Burkitt, Expository Notes With Practical Observations on the New Testament (Philadelphia: Published by Thomas Wardle, 1835), 2:742. [Italics original; underlining mine.]