From John Gill's commentary:
...and which is the constant character and practice of such men, who lie in wait to deceive, creep into churches at unawares, and into houses privately; and insinuate their principles under specious pretences and appearances of truth, using the hidden things of dishonesty, walking in craftiness, handling the word of God deceitfully, and colouring things with false glosses and feigned words: and even denying the Lord that bought them; not the Lord Jesus Christ, but God the Father; for the word κυριος is not here used, which always is where Christ is spoken of as the Lord, but δεσποτης; and which is expressive of the power which masters have over their servants (i), and which God has over all mankind; and wherever this word is elsewhere used, it is spoken of God the Father, whenever applied to a divine person, as in Luk_2:29 and especially this appears to be the sense, from the parallel text in Jud_1:4 where the Lord God denied by those men is manifestly distinguished from our Lord Jesus Christ, and by whom these persons are said to be bought: the meaning is not that they were redeemed by the blood of Christ, for Christ is not intended; and besides, whenever redemption by Christ is spoken of, the price is usually mentioned, or some circumstance or another which fully determines the sense; see Act_20:28 whereas here is not the least hint of anything of this kind: add to this, that such who are redeemed by Christ are the elect of God only, the people of Christ, his sheep and friends, and church, and who are never left to deny him so as to perish eternally; for could such be lost, or deceive, or be deceived finally and totally by damnable heresies, and bring on themselves swift destruction, Christ's purchase would be in vain, and the ransom price be paid for nought; but the word "bought" regards temporal mercies and deliverance, which these men enjoyed, and is used as an aggravation of their sin in denying the Lord; both by words, delivering out such tenets as are derogatory to the glory of the divine perfections, and which deny one or other of them, and of his purposes, providence, promises, and truths; and by works, turning the doctrine of the grace of God into lasciviousness, being disobedient and reprobate to every good work; that they should act this part against the Lord who had made them, and upheld them in their beings and took care of them in his providence, and had followed them with goodness and mercy all the days of their lives; just as Moses aggravates the ingratitude of the Jews in Deu_32:6 from whence this phrase is borrowed, and to which it manifestly refers: "do ye thus requite the Lord, O foolish people and unwise! is not he thy Father that hath bought thee? hath he not made thee, and established thee?" nor is this the only place the apostle refers to in this chapter, see 2Pe_2:12 compared with Deu_32:5 and it is to be observed, that the persons he writes to were Jews, who were called the people the Lord had redeemed and purchased, Exo_15:13 and so were the first false teachers that rose up among them; and therefore this phrase is very applicable to them:
