"The slaughter and great carnage Which have with crimson stained the Arbia, cause Such orisons in our temple to be made"

(Canto 10, lines 85-86)

The "slaughter and great carnage" here refers to the Battle of Montaperti, where the Ghibelline forces were victorious - Arbia is a river in the vicinity. At the time, decisions such as persecuting the Ghibellines for revenge would have taken place in the church, probably in the Florence Baptistery.