The New Testament keeps and urges this office of the Law, as St. Paul does when he says, "The wrath of God is revealed from heaven against all ungodliness and unrighteousness of men" (Romans 1:18). Also, "the whole world may be accountable to God ... No human being will be justified in His sight" (Romans 3:19-20). And, Christ says, the Holy Spirit will convict the world of sin.
This is God's thunderbolt. By the Law He strikes down both obvious sinners and false saints. He declares no one to be in the right, but drives them all together to terror and despair. This is the hammer. As Jeremiah says, "Is not My word like ... a hammer that breaks the rock in pieces?" (23:29). This is not active contrition or manufactured repentance. It is passive contrition, true sorrow of heart, suffering, and the sensation of death.
This is what true repentance means. Here a person needs to hear something like this, "You are all of no account, whether you are obvious sinners or saints (in your own opinions). You have to become different from what you are now. You have to act differently than you are now acting, whether you are as great, wise, powerful, and holy as you can be. Here no one is godly."
But to this office of the Law, the New Testament immediately adds the consoling promise of grace through the Gospel. This must be believed. As Christ declares, "Repent and believe in the gospel" (Mark 1:15). That is become different, act differently, and believe My promise. John the Baptist (preceding Christ) is called a preacher of repentance, but this is for the forgiveness of sins. That is, John was to accuse all and convict them of being sinners. This is so they can know what they are before God and acknowledge that they are lost. So they can be prepared for the Lord to receive grace and to expect and accept from Him the forgiveness of sins. This is what Christ Himself says, "Repentance and forgiveness of sins should be proclaimed in [My] name to all nations" (Luke 24:47).
~BOC, SA, III, III, 1-6