While they promise them liberty - True religion always promises and produces liberty (see the notes at John 8:36), but the particular liberty which these persons seem to have promised, was freedom from what they regarded as needless restraint, or from strict and narrow views of religion.
They themselves are the servants of corruption - They are the slaves of gross and corrupt passions, themselves utter strangers to freedom, and bound in the chains of servitude. These passions and appetites have obtained the entire mastery over them, and brought them into the severest bondage. This is often the case with those who deride the restraints of serious piety. They are themselves the slaves of appetite, or of the rules of fashionable life, or of the laws of honor, or of vicious indulgences. “He is a freeman whom the truth makes free, and all are slaves besides.” Compare the notes at 2 Corinthians 3:17.
For of whom a man is overcome ... - Or rather “by what (ᾧ hō) anyone is overcome;” that is, “whatever” gets the mastery of him, whether it be avarice, or sensuality, or pride, or any form of error. See the notes at Romans 6:16, where this sentiment is explained.