Thomas Agni of Leontino, Dominican archbishop of Cosenza, and later patriarch of Jerusalem, was the first to write a life of the blessed martyr. He lived for many years with Peter of Verona and had been his superior.
He was born in the city of Verona into a family perhaps sympathetic to the Cathar heresy. Peter went to a Catholic school, and later to the University of Bologna, where he is said to have maintained his orthodoxy and at the age of fifteen, met Saint Dominic. Peter joined the Order of the Friars Preachers (Dominicans) and became a celebrated preacher throughout northern and central Italy.
From the 1230s on, Peter preached against heresy, and especially Catharism, which had many adherents in thirteenth-century Northern Italy. Pope Gregory IX appointed him General Inquisitor for northern Italy in 1234. and Peter evangelized nearly the whole of Italy, preaching in Rome, Florence, Bologna, Genoa, and Como. In 1243 he recommended the new Servite foundation to the pope for approval.
In 1251, Pope Innocent IV recognized Peter's virtues (severity of life and doctrine, talent for preaching, and zeal for the orthodox Catholic faith), and appointed him Inquisitor in Lombardy. He spent about six months in that office and it is unclear whether he was ever involved in any trials. His one recorded act was a declaration of clemency for those confessing heresy or sympathy to heresy.
In his sermons he denounced heresy and also those Catholics who professed the Faith by words, but acted contrary to it in deeds. Crowds came to meet him and followed him; conversions were numerous, including many Cathars who returned to orthodoxy.
Because of this, a group of Milanese Cathars conspired to kill him. They hired an assassin, one Carino of Balsamo. Carino's accomplice was Manfredo Clitoro of Giussano. On April 6, 1252, when Peter was returning from Como to Milan, the two assassins followed Peter to a lonely spot near Barlassina, and there killed him and mortally wounded his companion, a fellow friar named Dominic.
Carino struck Peter's head with an axe and then attacked Domenico. Peter rose to his knees, and recited the first article of the Symbol of the Apostles (the Apostle's Creed). Offering his blood as a sacrifice to God, according to legend, he dipped his fingers in it and wrote on the ground: "Credo in Deum", the first words of the Apostles' Creed. The blow that killed him cut off the top of his head, but the testimony given at the inquest into his death confirms that he began reciting the Creed when he was attacked.