Actually, if you want to be technical, infidel is a relative term. When spoken by a Muslim, it means non-Muslim. When spoken by a Christian, it means non-Christian. And so on, and so forth; the basic meaning is "one who does not follow the religious beliefs of the point of view the speaker is using".

Currently, it seems to be a fashionable term among fundamentalist Muslims. It was popular in Christianity too, for a while.

You know what's interesting? Every religion seems to go through this militaristic phase when it gets to be about 1300-1500 years old. Islam's going through it now. Christianity went through it at that age. If we reach back through the Old Testament, Judaism would appear to have gone through a similar phase. I'm not up enough on the history of Hinduism to know if it did (but I'd guess it probably did). I don't think Buddhism as we currently know it is old enough to have gone through this particular phase, though it's had its own share of warriors in the past. I wonder why this is. Why 1500 years?