Hildabrand aka Pope Gregory VII – Wikipedia

One of the great reforming popes, he is perhaps best known for the part he played in the Investiture Controversy, his dispute with Emperor Henry IV to establish the primacy of papal authority and the new canon law governing the election of the pope by the College of Cardinals. He was also at the forefront of developments in the relationship between the emperor and the papacy during the years before he became pope. He was the first pope to introduce a policy of obligatory celibacy for the clergy, which had until then commonly married,[2][3][4][5] and also attacked the practice of simony.
— Read on en.m.wikipedia.org/wiki/Pope_Gregory_VII