Here is Dr. Schaeffer's best book in just a few lines. In defense of Christian faith, there are 3 essential problems, just to get started:
1, the problem of existence. Why does anything exist? The fact that things do exist is a huge (foundational) problem for the conventional modern person, although they seldom see it.
2, the problem of evil. Is there an unbroken line of continuity between God and evil, or is God mad that evil exists, too?
3, the problem of knowing. Is it possible to know our world and God truly, even though we can't know everything? Why?
This is what the excellent book HE IS THERE AND HE IS NOT SILENT covers. But each time, he shows that as you start to look at the Christian message in entirety, it has none of them! This is a problem for the modern person, foundationally, because there is no where to go on any of them. They try materialism (in C S Lewis' sense) or Bhuddism/pantheism, very unhappily.
Ted T's post is about #2 of course.