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Pope Gregory IX organized the Pontifical Inquisition with the aim of repressing heresies, with the promulgation of two documents, of which the second, the "Licet ad capiendos" of April 20, 1233, which was addressed to the Dominicans, who began to lead the work of investigation, trial, condemnation and absolution of "heretics." This second document officially established the Inquisition astoday is known.
"Wherever it occurs to them [heretics] to preach, you are empowered, if sinners persist in advocating heresy in spite of warnings, to deprive them forever of their spiritual benefits and to proceed against them and all others, without appeal, requesting in case of need the assistance of secular authorities and overcoming their opposition, if this is necessary, by censureecclesiastical unappealable".
Translated excerpt from the "Licet ad capiendos" of 1233, which established the Inquisition
Gregory IX, painting by Raphael in the Stanza della Segnatura, Vatican
Throughout six centuries of the Christian era, the Inquisition acted in barbaric ways, affecting the political, economic, social and cultural spheres of Christian society. The suspected heretic - who were mostly women accused of witchcraft - was often spied on by agents appointed by the Holy Office before being brought to trial. In court, the suspect was interrogated by threeinquisitors, one of whom was the chief inquisitor, responsible for the final sentence.
Cats and the Inquisition
Humans were not the only victims of the persecution. Pope Gregory IX also issued the papal bull Vox in Rama After the decree, a massacre of cats began, which from then on were seen as demonic, or the very personification of the devil. It did not take long for people to start killing and torturing cats, especially black ones. It is possible that the idea that black cats are bad luck, may have arisen from thisterrible episode of religious fanaticism.
Illustration from a manuscript produced in Brittany, dated c. 1430 - c. 1440, depicting hunters with dogs shooting arrows at a wildcat in a tree.
The outbreak of the Black Death in Europe
With cats being slaughtered due to unfounded religious beliefs, there was a huge explosion in the increase of the rat population throughout Europe. These rodents were ideal hosts for the fleas that harbor the bacteria Yersinia pestis, This was one of the factors that contributed to one of the most devastating pandemics in human history, the black plague that killed about 200 million people in the middle ages. The Inquisitors of the time ended up digging their own grave while preaching the death to those little cats. 
A mass grave of plague victims from the early 18th century in Martigues, France. Image: S. Tzortzis , CC BY
