Tim Keller on why apologetics is essential:
Apologetics is an answer to the “why” question after you’ve already answered the “what” question. The what question, of course, is, “What is the gospel?” But when you call people to believe in the gospel and they ask, “Why should I believe that?”—then you need apologetics.
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.