Steve McCoy has done the church a service by making available in one location everything Tim Keller.  One such article he links to is this one: All of Life is Repentance.  In this article Keller compares religious repentance to gospel repentance. 

Religious repentance, says Keller, is "selfish, self-righteous, and bitter all the way to the bottom”.  It is selfish because we only do it to avoid punishment.  It is self-righteous because we can be duped into doing it as a means of atonement.  And it is bitter because, “in religion our only hope is to live a good enough life for God to bless us.  Therefore every instance of sin and repentance is traumatic, unnatural, and horribly threatening.  Only under great duress does a religious person admit they have sinned because their only hope is their moral goodness.” 

Contrast this with gospel repentance that hates sin because of love for God.  Gospel repentance receives grace rather than trying to earn it through self punishment.  And finally a firm understanding of the gospel does not produce bitterness but rather joy in repentance.  As Keller nicely sums up, “The more you see your own flaws and sins, the more precious, electrifying, and amazing God’s grace appears to you.  But on the other hand, the more aware you are of God’s grace and acceptance in Christ, the more you [are] able to drop your denials and self-defenses and admit the true dimensions of your sin.”

When you understand the difference between religious repentance and gospel repentance then, and only then, can you begin “to profit from a regular and exacting discipline of self-examination and repentance”.  Keller then uses a statement by George Whitefield to put together a few questions to foster gospel-centered repentance:

• Deep Humility v. Pride
• Burning love v. indifference
• Wise courage v. anxiety
• Godly motivations (a single eye)

You’ll have to read the whole article to see how Keller expounds upon these…it’s only two pages and well worth your time.