Then, at the request of Guy of Pomposa (Guido d'Arezzo) and other heads of neighbouring monasteries, for two or three years he lectured to their brethren also, and (about 1042) wrote the life of Romuald for the monks of Pietrapertosa.

A zealot for monastic and clerical reform, he introduced a more severe discipline, including the practice of flagellation ("the disciplina") into the house, which, under his rule, quickly attained celebrity, and became a model for other foundations, even the great abbey of Monte Cassino.

There was much opposition outside his own circle to such extreme forms of penitence, but Peter's persistent advocacy ensured its acceptance, to such an extent that he was obliged later to moderate the imprudent zeal of some of his own hermits.

[2] Although living in the seclusion of the cloister, Peter Damian closely watched the fortunes of the church, and like his friend Hildebrand, the future Pope Gregory VII, he strove for reforms in a deplorable time.

He was present in Rome when Clement II crowned Henry III and his consort Agnes, and he also attended a synod held at the Lateran in the first days of 1047, in which decrees were passed against simony.

About 1050, he wrote Liber Gomorrhianus addressed to Pope Leo IX, containing a scathing indictment of the practice of simony, as threatening the integrity of the clergy.

Pope Benedict XVI described him as "one of the most significant figures of the 11th century, ... a lover of solitude and at the same time a fearless man of the Church, committed personally to the task of reform.

Toivo J. Holopainen identifies De divina omnipotentia as "an interesting document related to the early developments of medieval discussion concerning modalities and divine omnipotence".

For a long time, Damian resisted the offer, for he was more at ease as an itinerant hermit-preacher than as a reformer from within the Curia, but was finally forced to accept, and was consecrated Cardinal Bishop of Ostia on 30 November 1057.

The resistance of the clergy of Milan to the reform of Ariald the Deacon and Anselm of Lucca rendered a contest so bitter that an appeal was made to the Holy See.

[5] He exacted first a solemn oath from the archbishop and all his clergy that for the future no preferment should be paid for; then, imposing a penance on all who had been guilty, he reinstated in their benefices all who undertook to live in celibacy.

[5] In 1063 the pope held a synod at Rome, at which Peter Damian was appointed legate to settle the dispute between the Abbey of Cluny and the Bishop of Mâcon.

[citation needed] While he was in France the antipope Cadalous had again become active in his attempts to gain Rome, and Peter Damian brought upon himself a sharp reproof from Alexander and Hildebrand for twice imprudently appealing to the royal power to judge the case anew.

After a period of retirement at Fonte Avellana, he proceeded in 1069 as papal legate to Germany and persuaded the emperor Henry IV to give up his intention of divorcing his wife Bertha.

Peter Damian is venerated as a saint and was made a Doctor of the Church by Pope Leo XII on 27 September 1828 with a feast day which is now celebrated on 21 February (Ordinary calendar).

[citation needed] The saint is represented in art as a cardinal bearing a knotted rope (the disciplina) in his hand; also sometimes he is depicted as a pilgrim holding a papal Bull, to signify his many legations.

[citation needed] Peter Damian's voluminous writings, including treatises (67 survive), letters, sermons, prayers, hymns and liturgical texts (though, in a departure from many early medieval monks, no biblical commentaries)[10] reflect the spiritual conditions of Italy: the groundswell of intense personal piety that would overflow in the First Crusade at the end of the century, and his Latin abounds in denunciatory epithets.