i afraid I still have to disagree. When the crusades began there was no world power, England hadn't colonized the states yet. As far as the known world was concerned, all of civilization was over in Europe. The Kings of Europe were under the power of the pope still. The pope said jump and they asked, how high? When the ottoman empire spread into Spain (against the direction of the Quran I might add), this was at a time when all of Islam was on the offensive. The pope then sent for the Kings to send their armies to fight them back. All Christian men able to fight tried to join the crusade if they were able. It is documented that wives would lock their husbands in a room and not allow them to leave, keeping them prisoner. The church was very successful in recruiting ridiculous numbers into their army to fight Islam. This for me is the difference. The central and primarily (and at the time, ONLY) authority for all Christians in the then known world sanctioned, approved, and encouraged the crusades. They proscribed for their worshippers that joining the army was the greatest calling in the world, and of they did so all sins would be forgiven them. They did not direct this for only a few or a handful, but for all Christians in the known world of the day. This, to me, is vastly different from ISIS and there little handful of radicalized fools.
If the church were to try something like that today, it would not reflect on all Christianity because since then Christians habe splintered into 10s of 1000s of groups. What the Catholics do does not reflect on baptists or Methodists and vice versa. The same is true of ISIS, and other dempnominations of Islam. However, back at the time of the crusades there was only a single Christian denomination. Therefore, since that one denomination sanctioned and approved it, it reflects on all of them at that time and the Catholic Church today.
Atrocities like this, as well as the false teachings of Catholicism, is the reason for the reformation, and the development of all the different Christian groups today. For this reason, because they took action to sever themselves from this past, the crusades no longer reflect on all Christianity. Compared with Islam however, they are already splintered. If there had been other denominations of Christianity around during the crusades then the crusades would only reflect on the ones who took part and sanctioned them. The same should be true of Islam, which does not as a whole sanction and endorse ISIS. This just like the crusades would not reflect on multiple denominations if they had existed, ISIS does not reflect on Islam as a whole.
i feel like this response was long winded, but I'm hoping more words made me easier to understand. I'm not great at articulation.