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“Growth in character and changes in behavior occur in a gradual process after a person becomes a Christian. The mistaken belief that a person must “clean up” his or her own life in order to merit God’s presence is not Christianity. This means, though, that the church will be filled with immature and broken people who still have a long way to go emotionally, morally, and spiritually. As the saying has it: ‘The church is a hospital for sinners, not a museum for saints.'” —Tim Keller, “The Church is Responsible for So Much Injustice” from The Reason for God, p. 55
This quote really spoke to me when I read it. It was one of those quotes in which I sat back and thought, Wow. This is revolutionary. This is why Redeemer Presbyterian in New York City has been so successful. This statement is virtually contrary to what I experienced when I became a born-again Christian.
I entered Christian fundamentalism at 16. One of the key things stressed upon my conversion was repentance. I needed to immediately turn away from my sins and turn to God.
And I think this is one of the key things about Christianity that keeps many people away: not only do they not see a need to turn away from their sins, but even if they did, fundamental Christians capitalize on emotional momentum and force them to “make a decision.”
What I like about what Mr. Keller says here is that full repentance—turning away from sin and to God—is a gradual process. Is it a requirement to be sorry for one’s sins and living in disobedience against God upon conversion? Absolutely. But to expect instantaneous change from a new believer is wrong. If instant change happens, that’s nice but no expectation of immediate change should be placed upon the new believer (which is something that happens all too often). The new believer should be discipled and bathed in the words of the Bible to be able to come to an understanding on his or her own of what God requires. Out of that understanding, through God’s love, and the leading of the Holy Spirit, will a new believer be able to gain ground to turn away from sin. While mature believers should counsel younger ones in the faith in love and according to Scripture, no one likes to be told what to do from someone with a smug and judgmental attitude.
Overall, Mr. Keller’s chapter on “The Church is Responsible for So Much Injustice” in The Reason for God gives great insight into why Christians seem to suffer from gross moral failings opposed to their irreligious counterparts. (A trend I’ve noticed but have always wondered about.)
This Journey Is My Own 
Your post mentions and tweets are making me want to read this book. Keller seems like a guy who sees Christianity the way I’ve found scripture to present it, as opposed to the lists of rules and all that’s too often seen in American (particularly conservative) Christianity.
Tim Keller is, without a doubt, one of the most practical Christians I have ever read. He is not high and lofty; he’s down-to-earth and really relates the current challenges of life to a holy book that often seems antiquated. The Reason for God is a fine place to start. I can lend you the book when I’m done if you like.