All the text that is on the first page that pops up when you choose a civilopedia entry is found in pedia.txt. All the text found when you hit the various "description" buttons is in describe.txt (MGE).
that isn't what you want. You probably already changed the name to facism in rules.txt; but there are also changes to be made in game.txt and labels.txt. Search both of those for "fundamentalism" and change them. That should take care of most of the occurences that pop up throughout the game.
Forget about the one in game.txt that's just for the cheat menu. The line you want to change is in labels.txt it say "fundamentalism only" change that to "fascism only"