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.]

Hardy:

The more to enforce this upon us, take notice,

1. Who it is, the Son, and that in a double notion.

(1.) Being the Son, he ‘thinketh it no robbery to be equal with God,’ Philip, iii. 5, inasmuch as, according to the Athanasian creed, he is God of God, light of light, very God of very God. Being the Son, he is heir of all things. Lord of heaven and earth; and shall we in any kind, or for any cause, deny him? This is that which St Jude brings in as an aggravation of the sin of these very antichrists, whom he calls ‘certain men crept in unawares, they denied the only Lord God, and our Lord Jesus Christ,’ Jude 4; where, though some take the words disjunctively, applying the first clause to the Father, and the second to the Son, yet since there is no article in the Greek between theon and kurion, God and Lord, to divide them; yea, the word despotes, in that parallel place of St Peter, is evidently used of Christ, and withal the heresies of those times more directly struck at Christ than at God the Father; it is not improbable that St Jude intended here only to set forth Christ in his natures and prerogatives, whom he calls ‘the only Lord God’ (as elsewhere the Father is styled ‘the only true God’), not in exclusion of the other persons, but of all false deities. And now, when we set before us the divinity, majesty, sovereignty, and authority of Christ, the only Lord God, how must the sin of denying him appear beyond measure sinful!

(2.) This glorious and eternal Son of God was pleased to undertake and accomplish the work of our redemption, and it would be no other than a monstrous ingratitude to deny him. Upon this account, St Peter, speaking of these very antichrists under the name of false teachers, 2 Pet. ii. 2, aggravates their denial of Christ, in that it was of ‘the Lord that bought them.’ There cannot be a more execrable villainy, than for a slave to disown his lord that hath ransomed him. Who would not cry shame on that son who should deny his own father? And may I not say of the Son of God, in Moses his language, to every one of us, Deut. xxxii. 6, ‘Is not he thy Father that hath bought thee?’ ‘What is there thou canst be in danger of by acknowledging him, which he did not actually undergo to redeem thee? Is it loss of estate? he was poor; of credit? he was reviled; of liberty? he was bound; of life? he was crucified; and shall any of these dishearten us from honoring, or induce us to deny him? When therefore any temptations shall assault us (as once they did Peter) to deny him, let us remember what he is in himself, and what he hath done for us; let us consider his greatness, and be afraid; his goodness, and be ashamed; for fear, or shame, or any cause whatsoever to deny him.

2. That I may drive the nail to the head, let us often set before our eyes that dismal communication so often denounced in the Gospel by the Son of God himself against those who shall deny him. Mat. x. 33, ‘Whosoever shall deny me before men, him will I also deny before my Father which is in heaven;’ and again inculcated by St Paul, 2 Tim. ii. 13, ‘If we deny him, he will deny us:’ a threat than which none more just, and yet withal none more terrible. Just it is, in that it is the retaliation of like for like. What more rational than that despisers should be despised, forsakers should be forsaken, and deniers should be denied? And how terrible it is will soon appear, if you consider that the Son of God will then deny us, when he shall appear in his glory, that he will deny us not only before men, but angels, nay, his Father; that if he pronounce upon us an know you not (which is to deny us), we are the cursed of the Father; he will not acknowledge them for his adopted children, who durst not here own his begotten Son, and whom his Son will not then own for brethren; yea, which consummates the misery of such apostates, they must ‘have their portion with hypocrites,’ having denied Christ; and being denied by him, they must depart from him into that fire which is prepared for the devil and his angels, there being no reason that they should be near to Christ hereafter, who follow him afar off, nay, run away from him here.

Nathanael Hardy, The First General Epistle of St John the Apostle, Unfolded and Applied (Edinburgh: James Nichol, 1865), 344.   [Some spelling modernized; underlining mine.] [Note: Of further interest here is that Hardy’s thinking here predates the Granville Sharp rule by about one century.]

2 Peter 2:1:

Denying] By total apostasy, or evil life, unbelieving the servants of Christ, Tit. 1:16. See more on Jude 4.

The Lord that bought them] That gave a price sufficient for them, even his own precious Blood, Acts, 20:27; 1 Cor. 6:20; 1 Pet. 1:18, 19. Or, by whom they professed that they were redeemed: and therefore they should not have denied him.

Annotations Upon all the Books of the Old and New Testament: This Second Edition so enlarged, As they make an entire Commentary on the Sacred Scripture: The like never before published in English. Wherein The Text is Explained, Doubts Resolved, Scriptures Paralleled (London: Printed by John Legat, 1651). [No pagination.]

Jude 4:

ungodly men] Such as worship God not aright; or have no fear of God at all, Gen. 20:11; Psa. 30:1; Rom. 3;18.

turning the grace of our God into lasciviousness] The grace of God invites us to sobriety, Tit. 2:11,12, but they turn it to a contrary end.

Denying the only Lord God, and our Lord Jesus Christ] Denying Christ to be God, who was their master by profession (for the professed themselves to be of his household) and their Lord by public authority over them. Or, by their deeds denying Christ.

Annotations Upon all the Books of the Old and New Testament: This Second Edition so enlarged, As they make an entire Commentary on the Sacred Scripture: The like never before published in English. Wherein  The Text is Explained, Doubts Resolved, Scriptures Paralleled (London: Printed by John Legat, 1651). [No pagination.]

[Notes: These two brief comments demonstrate that the early Reformed theology was able to interpret these verses in such a way that Christ, as Lord and Master, is the subject, while also allowing that the “buying” was by way of a sufficient redemptive price for them.]

Willet:

And deny God the only Lord, and our Lord Jesus Christ]

These words thus translated seem to speak of two persons, of God the Father, and God the Son: but indeed the whole sentence is to be understood of Christ, who is called God, and despotes, master, and Kurios, Lord: so that Lord here in the first place should be translated master: for Christ is God, in respect of his Godhead with his Father: he is our master, because he has bought us, 2 Peter 2:1, he is our Lord, because by him all things are preserved, 1 Cor. 8:6, Heb. 1:3, so that he is God as our creature, Lord as our preserver, and master as our redeemer.

Andrew Willet, A Catholicon, that is, A general preservative or remedie against Pseudocatholike religion gathered out of the Catholike epistle of S. IVDE, (Printed by Iohn Legat, Printer to the University of Cambridge, and are to be sold at the sign of the Crowne in Pauls Churchyard by Simon Waterton, 1602), 23.   [Some spelling modernized, underlining mine.]

[Note: Willet here reflects the Christological reading of 2 Peter 2:1 and Jude 4 which was the more general and received exegetical position in early Reformed theology.]