2 Peter 2:19

While they promise them liberty - Either to live in the highest degrees of spiritual good, or a freedom from the Roman yoke; or from the yoke of the law, or what they might term needless restraints. Their own conduct showed the falsity of their system; for they were slaves to every disgraceful lust.

For of whom a man is overcome - This is an allusion to the ancient custom of selling for slaves those whom they had conquered and captivated in war. The ancient law was, that a man might either kill him whom he overcame in battle, or keep him for a slave. These were called servi , slaves, from the verb servare , to keep or preserve. And they were also called mancipia , from manu capiuntur , they are taken captive by the hand of their enemy. Thus the person who is overcome by his lusts is represented as being the slave of those lusts. See Romans 6:16 , and the note there.