Jesus answered them, “Truly, truly, I say to you, everyone who practices sin is a slave to sin.
34. Every man who committeth sin is the slave of sin. This is an argument drawn from contrary things. They boasted that they were free. He proves that they are the slaves of sin, because, being enslaved by the desires of the flesh, they continually sin. It is astonishing that men are not convinced by their own experience, so that, laying aside their pride, they may learn to be humble.
We have in these verses, I. A comfortable doctrine laid down concerning the spiritual liberty of Christ's disciples, intended for the encouragement of those Jews that believed. Christ, knowing that his doctrine began to work upon some of his hearers, and perceiving that virtue had gone out of him, turned his discourse from the proud Pharisees, and addressed himself to those weak believers.
Commenting on John 8:31-37
I know that ye are Abraham's seed,.... In answer to the other part of the Jews' objection to Christ, and in favour of themselves, Christ owns that they were the natural seed of Abraham; for truth must be allowed to an adversary.