Thanks to Aaron Denlinger:
[John] Craig’s teaching role in the Dominican monastery granted him access to the library of the Roman Inquisition, and at some point in the 1550s, while taking advantage of that privilege, he stumbled across an early edition of John Calvin’s Institutes of the Christian Religion. Craig read Calvin’s work and embraced the doctrine he found therein, which, needless to say, put him in an awkward spot. Sometime shortly after this he apparently confessed his evangelical convictions to an elderly monk in his order. The aged monk responded that he shared Craig’s sympathy towards reforming ideas, but encouraged Craig to keep his mouth shut for his own safety.
But Craig, rightfully overjoyed at his new-found understanding of the gospel, couldn’t quite manage that task. His desire to tell others what he had discovered eventually led to his arrest and trial at the hands of that very institution — the Roman Inquisition — that had unwittingly provided him access to the reforming views of Calvin. Craig was found guilty of heresy and imprisoned in a cell in the basement of the Roman Palazzo dell’ Inquisizone. The Palazzo apparently bordered the Tiber, which at that time lacked the stone embankments which today keep the river in check, and Craig’s cell, according to one source, regularly filled with waist-high water, adding considerably (one imagines) to the unpleasantness of imprisonment and impending death.
Following nine months of imprisonment, Craig’s execution date was set for August 19th. The evening before he was scheduled to die, however, Pope Paul IV, who had been instrumental in the establishment of the Roman Inquisition in the 1540s and had, as Pope, considerably inflated its authority and activities, died. Paul IV was a decidedly unpopular person in Rome, not least because of the far-reaching powers he had given the Inquisition. As news of his death circulated, the Roman people naturally convened outdoors to celebrate. They subjected Paul IV’s recently erected statue in the Piazza del Campidoglio to a mock trial and, having found the same guilty of one thing or another, decapitated the marble Pope, dragged his body through the streets, and finally cast him into the river. Their taste for rioting and revenge having merely been whetted, they then sacked (and eventually burned) the Palazzo dell’ Inquisizone, murdered the resident Inquisitor and beat up his underlings, and freed seventy-plus persons who were currently imprisoned in the Palazzo’s cells, including Craig.