Galatians 3:19

Galatians 3:19. It was added because of transgressions, till the seed should come to whom the promise was made;

The law makes us know what transgression is; it reveals its true nature. Under the hand of the Holy Spirit, it makes us see the evil of sin. We might not have perceived sin to be sin if it had not been for the command of God not to commit it; but when the commandment comes, then we recognize sin and the evil of it.