This is a common mistake that is made. The Catholic church was the dominant church at the time of the Crusades, but it's actions were not necessarily supported by Scripture. I have no problem with people/nations defending themselves from an aggressive barbaric enemy, but nowhere in Scripture (which is the rule of our faith in Christianity) does it tells us, or a church, to physically attack and convert non-believers. So taking an action of a church, which was also a part of the political structure, and extrapolating that it's behavior was Christian is wrong.
And I think
that is a common mistake that is made.
I was not saying that the Crusades were
compelled by Christianity, or even that they were consistent with biblical teaching. But in the context of the title of this thread, can it be said that "the Crusades had
nothing to do with Christianity"? Of course not. Whether through a twisted interpretation or not, the fact that they were called by the Pope and done in the name of Christianity meant they had 'something" to do with Christianity.
The same is true of Islamic terrorism. Whether particularly Muslims disagree with those religious interpretations isn't the point. The fact that we have to argue religious interpretations at all means that the current wave of terrorism does have "something to do" with Islam. It's inarguable.