Witches can kill your priests if they are unsuccessful at denoucing them. Basically, it says something like your priest was found dead , like he had been burned by fire.

The Theological Guilds are very handy for producing priests who are less likely to become heretical. They also have a higher rate of denouncement?. This is one way to quickly stack the College of Cardinals playing as any Catholic faction. Each successful denoucement gives a better chance to raise piety.