Calvin:

Though Christ may be denied in various ways, yet Peter, as I think, refers here to what is expressed by Jude, that is, when the grace of God is turned into lasciviousness; for Christ redeemed us, that he might have a people separated from all the pollutions of the world, and devoted to holiness ,and innocency. They, then, who throw off the bridle, and give themselves up to all kinds of licentiousness, are not unjustly said to deny Christ by whom they have been redeemed. Calvin, 2 Peter 2:1.

“The only Lord God,” or, God who alone is Lord. Some old copies have, “Christ, who alone is God and Lord.” And, indeed, in the Second Epistle of Peter, Christ alone is mentioned, and there he is called Lord. But He means that Christ is denied, when they who had been redeemed by his blood, become again the vassals of the Devil, and thus render void as far as they can that incomparable price. Calvin, Jude 4.