This chapter consists of a strong denunciation of false teachers. They are, among other things, sensuous, beguiling, covetous, and accursed (v. 14). They desire personal wealth (vv. 15-16), but their message is empty, and even destructive, and will be judged (v. 17), appealing to the pride and lusts of their hearers (v. 18).
In our text we see the false teachers are quick to make promises. Promises are cheap; they cost nothing. Satan first revealed himself to mankind with a promise: "Ye shall be as gods" (Genesis 3:5), and later attempted to seduce the Son of God with "all the kingdoms of the world" (Matthew 4:8). Empty promises are Satan's golden hook, and many are the foolish ones who take the bait.
In this case, the false teachers promise liberty--liberty to act without the shackles of responsibility and moral law. But they themselves are "servants of corruption," slaves of a most abhorrent mentality. And who are they to offer liberty? These are indeed "great swelling words of vanity" (2 Peter 2:18), for slaves cannot rightly offer liberty.
How is this promise kept? Bondage. Bondage to that which has overcome. The liberty that sin promises is slavery, and the greater the sinner, the greater the bondage to the sin. There is perhaps no more wretched a state than to be in bondage to abject corruption in the name of liberty. It is a bondage of the spirit; a captivity of the soul. Of all states of slavery, it is the most lasting.
On the other hand, through grace we can "stand fast therefore in the liberty wherewith Christ hath made us free," with no need to be "entangled again with the yoke of bondage" (Galatians 5:1). JDM