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 KellerLove without truth is sentimentality; it supports and affirms us but keeps us in denial about our flaws. Truth without love is harshness; it gives us information but in such a way that we cannot really hear it. God's saving love in Christ, however, is marked by both radical truthfulness about who we are and yet also radical, unconditional commitment to us. The merciful commitment strengthens us to see the truth about ourselves and repent. The conviction and repentance moves us to cling to and rest in God's mercy and grace.
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