The fourth Pope of this name codified the Tridentine Creed while presiding over the last session of the Council of (*) Trent.
A Pope with this name declared that marriages between Catholics and Protestants were invalid unless they were performed in a Catholic Church or with a promise to raise the children as Catholic in the bull Ne temere. A different Pope with this name served as the adopted father of Edgardo Mortara, a Jewish child who was taken from his parents because a servant had baptized him. The fourth Pope of this name codified the Tridentine Creed while presiding over the last session of the Council of (*) Trent. The longest-serving Pope with this name wrote the Syllabus of Errors and defined the doctrine of papal infallibility. The last Pope with this name was criticized as the "Pope of Silence," due to his lack of action against the Holocaust. For 10 points, give this papal name of twelve Popes, the ninth of whom was the longest serving Pope in history.
A Pope with this name declared that marriages between Catholics and Protestants were invalid unless they were performed in a Catholic Church or with a promise to raise the children as Catholic in the bull Ne temere.
 Pius [number does not matter, so accept Pius I through Pius XII]