The following excerpt is from Tim Keller and J. Allen Thompson’s Church Planter Manual (New York: Redeemer Presbyterian Church, 2002), 190:
Without an orientation to the gospel, the heart will repent out of fear of consequences and out of fear of rejection. “Obey or you will be rejected.” But the gospel leads you to repent because Jesus died for your sin, so you would not be rejected. In a sense, the gospel says, “How can you treat one like this who paid this cost so that you would not be rejected?” Legalistic remorse says, “I broke God’s rules,” while real repentance says, “I broke God’s heart.”
Legalistic repentance takes sin to Mt. Sinai, gospel repentance to Mt. Calvary. Legalistic repentance is convicted by punishment, gospel repentance becomes convicted by mercy. Repentance out of mere fear is really sorrow for the consequences of sin, sorrow over the danger of sin – it bends the will away from sin, but the heart still clings. But repentance out of conviction over mercy is really sorrow over sin, sorrow over the grievousness of sin – it melts the heart away from sin. It makes the sin itself disgusting to us, so it loses its attractive power over us. We say, “This disgusting thing is an affront to the one who died for me. I’m continuing to stab him with it!”
Look at how Paul calls people to live moral lives. “For the grace of God that brings salvation has appeared to all men. It teaches us to say ‘no’ to ungodliness and worldly passions, and to live self-controlled, upright and godly lives” (Titus 2:11-12). Contrast this with how many sermons you have heard telling people to say “no” to immorality. Often the implicit or explicit reasons are: “It is against the Bible” or “it will hurt your self-esteem” or “it’s against our Christian principles” or “your sins will find you out; you’ll reap what you sow.” Often all of those things are true, but they are inadequate and secondary motives. Only the grace of God, the logic of the gospel will work. Paul says it “teaches” us to say no, it argues with us. The gospel tell you that the sin beneath your sins is that you have made something besides Christ the center of your life. You have concocted a self-salvation strategy based on something that you have decided is more important than Christ and more of a savior than he. The gospel tells you that your sin is always the result of disbelief that you are accepted in Christ alone.
The gospel creates the only kind of grief over sin which is clean and which does not crush. It says: “Look at Jesus dying for you! He won’t leave you or abandon you – how then can you respond as you are? He suffered so you wouldn’t do this thing! You are not living as though you are loved! As his child! It is not because he will abandon you that you should be holy, but because this is one who at inestimable cost to himself has said he won’t ever abandon you! How can you live in the very sin that he was ripped to pieces to deliver you from?” See the GRACE of God argument? It is the only argument which cannot be answered. This creates the only motivation that leads you to hate the sin without hating yourself. It is the only motivation that will bring sin to lose its attractive power over you.
How can this be? The sight of Christ dying for you is at once both the one thing in the world that most convicts you to be holy and yet most assures you that you are infallibly loved. If he died for you – that is the conviction. But if he died for you – that is the comfort.