Uttered by Salvatore, and dismissed in the movie as a way cry, what is the actual translation? I read the book a few times some years ago and cannot remember if it is defined at all. I know sparse Latin and I hope someone can help me out. Thanks!
Penitentiam agite was the cry of the followers of the Gherardo Segarelli, the founder of the Apostolici. The Church at the time was corrupted by greed, selling salvation, and two previous "rebellions" against it had been assimilated: the Benedictines (6th to 9th century, until they also became corrupt by wealth and power) and the Franciscans (13th-14th century, who were also becoming more and more compromising with the Pope).
Segarelli, angry at the spreading corruption and lure of comfort, money and power, started a new revolt with wandering priests, and their command was not just to preach about penitentia (repentance), but to also act in penitentia. That's why their motto penitentiam agite, that translated means "act in repentance" and not just talk about it.