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 when 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.
Is Apologetics Biblical?
There are plenty of Christians today who nevertheless say: "Don't do apologetics, just expound the Word of God---preach and the power of the Word will strike people." Others argue that "belonging comes before believing." They say apologetics is a rational, Enlightenment approach, not a biblical one. People need to be brought into a community where they can see our love and our deeds, experience worship, have their imaginations captured, and faith will become credible to them.
There is a certain merit to these arguments. It would indeed be overly rationalistic to say that we can prove Christianity so that any rational person would have to believe it. In fact, this approach dishonors the sovereignty of God by bowing to our autonomous human reason. Community and worship are important, because people come to conviction through a combination of heart and mind, a sense of need, thinking things out intellectually, and seeing it in community. But I have also seen many skeptics brought into a warm Christian community and still ask, "But why should I believe you and not an atheist or a Muslim?"
We need to be careful of saying, "Just believe," because what we're really saying is, "Believe because I say so." That sounds like a Nietzschean power play. That's very different from Paul, who reasoned, argued, and proved in the Book of Acts, and from Peter, who called us to give the reason for our hope in 2 Peter 3:15. If our response is, "Our beliefs may seem utterly irrational to you, but if you see how much we love one another then you'll want to believe too," then we'll sound like a cult. So we do need to do apologetics and answer the why question.
No Neutral Ground
However, the trouble with an exclusively rationalistic apologetic ("I'm going to prove to you that God exists, that Jesus is the Son of God, the Bible is true," etc.) is that it does, in a sense, put God on trial before supposedly neutral, perfectly rational people sitting objectively on the throne of Reason. That doesn't fit with what the Bible says about the reality of sin and the always prejudiced, distorted thinking produced by unbelief. On the other hand, an exclusively subjectivist apologetic ("Invite Jesus into your life and he'll solve all your problems, but I can't give you any good reasons, just trust with your heart") also fails to bring conviction of real sin or of need.
There will be no joy in the grace of Jesus unless people see they're lost. Thus a gospel-shaped apologetic must not simply present Christianity, it must also challenge the non-believer's worldview and show where it, and they, have a real problem.
Editors' Note: This is a cross-post from Tim Keller's blog at Redeemer City to City.