As you attack Catholic countries, the Pope will call upon other Catholic countries to fight you, even caling for Crusades against you.
For a tougher fight, here is an easy modding tip:
Go into the following folder, and open the file for the time period you are playing (such as Early, High, etc):
C:\Program Files\Total War\Medieval - Total War\campmap\startpos
Then:
1) Set unlimited end date:
SetStartDate:: 1087
SetPeriod:: EARLY
SetEarlyPeriodStartDate:: 1087
SetHighPeriodStartDate:: 1205
SetLatePeriodStartDate:: 1321
SetGameEndDate:: 2003
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