Excommunicate

From EU3 Wiki

If you are Catholic, you are at risk of being excommunicated, but also have the opportunity to excommunicate your enemies. Whoever has cardinals under their control, has a chance of becoming the Papal Controller. Whoever is the Papal Controller can excommunicate any Catholic ruler who has less then 100 relation with the Papal States (except the Papal States).

Excommunication is against the ruler, rather than the country, so if a ruler is part of a Personal Union, all countries in the union are affected.

Excommunication confers several penalties.

• -2.0 % a year to your prestige.
• A penalty to your tolerance for Catholicism.
• Rulers can declare war without the usual stability penalties for attacking within their religious group or without a casus belli.
• If a ruler dies while excommunicated, all unions dissolve at death.

If you are excommunicated, you may find that you are more likely to be attacked! For this reason, you should consider gifts to the papal state to maintain good relations if you intend to stay Catholic.