I've heard plenty of Christians try to answer the why question by going back to the what. "You have to believe because Jesus is the Son of God." But that's answering the why with more what. Increasingly we live in a time in which you can't avoid the why question. Just giving the what (for example, a vivid gospel presentation) worked in the days when the cultural institutions created an environment in which Christianity just felt true or at least honorable. But in a post-Christendom society, in the marketplace of ideas, you have to explain why this is true, or people will just dismiss it.
Timothy KellerForgiveness always comes at a cost to the one granting the forgiveness. To not retaliate is to absorb the cost.
Timothy KellerThis is the humbling truth that lies at the heart of Christianity. We love to be our own saviors. Our hearts love to manufacture glory for themselves. So we find messages of self-salvation extremely attractive, whether they are religious (Keep these rules and you earn eternal blessing) or secular (Grab hold of these things and youโll experience blessing now).
Timothy KellerWithout the gospel we hate ourselves instead of our sin. Without the gospel weโre motivated through all sorts of awful fear and pride to change and it doesnโt really change our hearts; it just restrains our hearts.
Timothy KellerJesus does not divide the world into the moral "good guys" and the immoral "bad guys". He shows us that everyone is dedicated to a project of self-salvation, to using God and others in order to get power and control for themselves. We are just going about it in different ways. Even though both sons are wrong, however, the father cares for them and invites them both back into his love and feast.
Timothy Keller