The big difference between a person who believes they’re a sinner saved by grace and a person who believes God has accepted them through their own efforts is if you have a moralistic, religious person, you say, ‘Are you a Christian?’ They say, ‘Well, of course. Why do you ask? Sure, I am. What do you mean? Aren’t you?’ You know?
If you ask a person who understands they’re holy and dearly loved, completely out of free grace, what they’re saying is, ‘Yeah, me a Christian, what a joke. Me? A Christian.’ There’s always a wonder or a playfulness. There’s an eternal surprise whenever you think of yourself, and you’re saying, ‘Me, of all people, a Christian? But it’s true. How could that be? Isn’t a joke? Isn’t it the great joke? Isn’t it the big Joke, with a capital J?’ Yet it’s true. Absolutely true. There’s a playfulness about Christians. There’s a wonder in them.
– Tim Keller
*Words found in Tim Keller’s 1989 Sermon, “Finding Our Identity in Christ — Part 3”