Excommunicate (or remit)
Some acts can get you excommunicated by the Pope. This is similar to being an Outlaw, except that you've messed with the Church instead of your King. Oftentimes being excommunicated means that the Pope will pressure your King to Outlaw you as well. Being excommunicated means that if you die without reconciling with the Church, you go to Hell. In addition, while you are excommunicated you cannot receive any of the sacraments, attend Church, obtain a divorce or a blessing, marry or have your children baptized. Your King or Overlord may also legitimately seize your properties if you are excommunicated.
There are many ways that you can run afoul of the Pope, but such things as pillaging, raiding or extorting Church owned property are sure ways to bring the wrath of His Holiness upon you. As well, killing or torturing Bishops or other NPCs of the Pope, or annoying an Overlord who is on good terms with the Pope can result in your excommunication.
The pope can remit the excommunication at any time he feels that the excommunicant has sufficiently repented his sins.