Pope Gregory IX was the head of the Catholic Church from March 19, 1227, until he died in 1241. He is known for issuing the Decretals of Gregory IX, medieval canon laws, and for instituting the papal inquisition, established in response to movements considered heretical to Roman Catholicism. Although he did many things during his reign as the Pope, he may be most known for his hatred of cats.
Throughout history, people have either loved or hated cats. This was no different. In medieval times, cats were sometimes seen as being symbols of Satan. On June 13, 1233, Pope Gregory IX issued a papal bull called Vox in Rama, which was part of Gregory’s crusade against heretics. This bull was in response to a finding found by Conrad von Marburg who, through torture and terror, reported that he had uncovered a satanic cult that worshiped many forms of the devil, including the black cat. This bull ordered those representing the church to seek out and destroy heretics.
Throughout the rest of the medieval era, cats were tortured and killed in astronomically large numbers. Although it is unknown how many cats were killed during this time, many historians estimate that so many cats were killed that it caused a wave of plague. Of course, people of that time believed that the plague was caused because Satan was furious at them for killing the cats. However, from a modern point of view, it is now widely known that fleas carried the plague. These fleas lived on rats. Cats generally killed those rats. Nevertheless, cats were now being killed in such large numbers that rat communities were thriving. Therefore, there was a plague.
Pope Gregory’s war on cats caused a tremendous amount of devastation.
 Thomas W. Smith, “The Use of the Bible in the Arengae of Pope Gregory IX’s Crusade Calls”, in Elizabeth Lapina and Nicholas Morton (eds.), The Uses of the Bible in Crusader Sources (Brill, 2017), 206–235.
 Donald W. Engles, Classical Cats: The Rise and Fall of the Sacred Cat (Milton Place, UK: Routledge, 2001), 183.