Here are a few paragraphs that were highlighted in my copy of Keller's book, The Reason For God. These paragraphs fall under Keller's heading, "The Threat of Grace":
Some years ago I met with a woman who began coming to church at Redeemer. She said that she had gone to church growing up and had never before heard a distinction drawn between the gospel and religion. She had always heard that God accepts us only if we are good enough. She said that the new message was scary. I asked her why it was scary, and she replied:
"If I was saved by my good works then there would be a limit to what God could ask of me or put me through. I would be like a taxpayer with "rights" - I would have done my duty and now I would deserve a certain quality of life. But if I am a sinner saved by sheer grace - then there's nothing he cannot ask of me."
She understood the dynamic of grace and gratitude. If when you have lost all fear of punishment you also lose all incentive to live a good, unselfish life, then the only incentive you ever had to live a decent life was fear. This woman could see immediately that the wonderful-beyond-belief teaching of salvation by sheer grace had an edge to it. She knew that if she was a sinner saved by grace, she was (if anything) more subject to the sovereign Lordship of God. She knew that if Jesus really had done all this for her, she would not be her own. She would joyfully, gratefully belong to Jesus, who provided all this for her at infinite cost to himself.
From the outside that might sound coercive, like a grinding obligation. From the inside the motivation is all joy. Think of what happens when you fall in love. Your love makes you eager for acceptance from the beloved. You ask, "Do you want to go out?" or maybe even, "Will you marry me?" What happens when the answer is "Yes"? Do you say, "Great! I'm in! Now I can act any way I want"? Of course not. No you don't even wait for the object of your affection to directly ask you do do something for them. You anticipate whatever pleases and delights them. There's no coercion or sense of obligation, yet your behavior has been radically changed by the mind and heart of the person you love.
- Tim Keller, The Reason For God, pg. 182-183