The Pope

[Myth]

Build and upgrade churches and grant him 5000 gold or more early game. When you go on your first crusade, bring 20 priests with you and mass convert islamic populations to Catholicism. This will promote said priests and in one generation time you will dominate 90% of the college of cardinals.

Alternatively - assassinate the pope when he gets too angry, the next guy will be more benevolent and might even lift your excommunication.