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.

The Gospel is the deepest consolation you can offer to the human heart.

Because sin is not just breaking the rules, it is putting yourself in the place of God as Savior, Lord, and Judge just as each son sought to displace the authority of the father in his own life.

God so loved us and hates suffering that he was willing to come down and get involved in it.

Describe the God you've rejected. Describe the God you don't believe in. Maybe I don't believe in that God either.

Honor the image of God. Human beings are not accidents, but creations.

Like a surgeon, friends cut you in order to heal you.