I don't think Catholicism could ever have been called a fundamentalist religion. It isn't completely based on one holy book (the Bible), it's always integrated other sources and teachings, such as the bit about Popes being infallible when making certain proclamations. As such, it's always evolving as a religion. Even more, Catholicism has always taken the stance that the Bible isn't something to be read and interpreted by regular people, and that only Church authorities can really interpret it correctly, which is why they always kept Masses in Latin and never translated the Bible to other languages. This was one of the issues in the Protestant Reformation, and why the King James Bible was such an important book at the time: it was the first time commoners were able to read it in their own language.

>it's almost like it takes a 1000 years, give or take, for a religion to blow off its steam and become reasonably peaceful.

Fundamental attribution error. It took Catholicism 1000 years to blow off that steam. That does not imply that the theology of Islam will ever move past barbarism.