Sin is what needs to be overcome in sanctification. So sin is the enemy in sanctification.
I go back to the Heidelberg Catechism, and at the end of the exposition of the Ten Commandments near the end of the Heidelberg Catechism, the question asks, “Why does God want this law to be preached so passionately?” And the answer is because He calls us to holiness even though He knows that the holiest of men in this life have only small beginnings of the obedience to which we’re called. So sin remains very much the reality in which we live and against which we struggle our whole Christian experience in this body.
So sin is the reality, and I think that question of the Catechism is so important because I think a lot of Christians are tempted to think, “Well, I’m doing pretty well. I’ve been regenerated. I’m a believer. I go to church. I’m really doing pretty well.” And the Catechism, I think very much in the spirit of the Bible and of the Reformation, says, “No, you’re really not doing that well.”
Sin is so much profounder a problem than we fully appreciate that we must be much more committed to the struggle of sanctification and to the seeking of the grace of God to help us.