During the Middle Ages, the Catholic Church held a lot of political power. The Pope was the one who was sent requests to legitimize royal blood, baptisms, etc. Furthermore, he was the one who blessed war-ships, etc. (if you recall the pope blessing the Spanish fleet of ships that went to fight England during the Protestant Revolution. The Pope could essentially make or break a royal court.
Secondly, the Catholic Church established many monasteries for monks and nuns, and pilgramages were widely popular in the Middle Ages. Hope this helps :)
