Falwell lists his justifications here, just for the record.

The real point, of course, is that it is largely irrelevant. Drawing conclusions about an entire modern religion based on its ancient history is misleading at best, and dangerous at worst.

I would hate to have modern Christianity judged by some of its representatives throughout history, including Biblical references. How many wives did King David and Solomon have? Granted Jesus comes off pretty good, but he's largely tacked on in a small bit between the end of the Old Testament and Paul's detailed interpretations of what Christianity means (if we were really accurate, the religion would probably be called Paulism.) If Jesus had as much independent documentation of his life as Mohammed did, there might be a bit more dirty laundry.

We need to deal with Islamic radicalism on its own merits and based on its own beliefs and actions. Some people have pointed out that a much larger percentage of Muslims are sympathetic to these views than radical outshoots of other religions. Maybe so, but it still doesn't pay to paint with too broad a brush.