Peter was "Dedicated to God" at birth and given to the monastery at Sauxillanges of the Congregation of Cluny. He took his vows there at age seventeen, swiftly rising in esteem and becoming professor and prior of the monastery of Vézelay at only twenty years of age. Later he went to the monastery at Domene. He was so successful in the fulfillment of his duties at Vézelay and Domene that by the age of thirty he was elected general of the order. Peter was a tireless advocate of reform within his order, which was in dire need of reconstruction after the deposing of the previous abbot, Pontius, by the pope. The Congregation of Cluny was also under attack by other orders and prominent monks and theologians, including St. Bernard of Clairvaux, a Cistercian monk. Successfully defending against these attacks, and completely reforming his order, Peter earned the appellation of "venerable."
Peter became a popular figure of the Roman Catholic Church, an international figure and associate of many national and religious leaders of his day. He attended many of the important religious councils of his generation, including the Council of Pisa in 1134, where he helped to avert a potential schism in the Church by supporting the cause of Pope Innocent II, and the Council of Reims in 1147. In addition, he defended French theologian Peter Abelard after the latter’s rationalistic Trinitarian interpretation had been condemned by the Council of Sens. He granted Abelard hospitality at Cluny and worked to mitigate the sentence of the council, eventually reconciling Abelard with his principal condemner, St. Bernard. After Abelard's death, Peter granted him absolution from his sins, at the personal request of Heloise.
Peter is well known for collecting sources on and writing about Islam (see below) and also as the author of vast amounts of correspondence, having authored letters on common theological questions, the Christian doctrine of the divinity of Christ, current heresies, and miracles. His writings are counted as some of the most important documents of the 12th century.
Despite his active life and important role in European history, Peter's greatest achievement stands his contribution to the reappraisal of the Church’s relations with the religion of Islam. A proponent of studying Islam based upon its own sources, he commissioned a comprehensive translation of Islamic source material, and in 1142 he traveled to Spain where he met his translators. One scholar has described this as a “momentous event in the intellectual history of Europe.”
The manuscripts concerned have been linked to Toledo which was an important centre for translation from the Arabic. However, Peter appears to have met his team of translators further north, possibly in La Rioja, where he is known to have visited Nájera. The project translated a number of texts relating to Islam (known collectively as the "corpus toletanum"). They include the Apology of al-Kindi; and most importantly the first-ever translation into Latin of the Arabic Qur'an (the "Lex Mahumet pseudoprophete"). Robert of Ketton was the main translator, Peter of Toledo is credited for planning and annotating the collection, and Peter of Poitiers helped to polish the final Latin version. The team also included Robert of Ketton's friend Herman of Carinthia and a Muslim called Mohammed. The translation was completed in either June or July 1143, in what has been described as “a landmark in Islamic Studies. With this translation, the West had for the first time an instrument for the serious study of Islam.”
Peter used the newly translated material in his own writings on Islam, of which the most important are the Summa totius heresis Saracenorum (The Summary of the Entire Heresy of the Saracens) and the Liber contra sectam sive heresim Saracenorum (The Refutation of the Sect or Heresy of the Saracens). In these works Peter portrays Islam as a Christian heresy that approaches paganism, and he explains to St. Bernard that his goal is "ut morem illum patrum sequerer, quo nullam unquam suorum temporum vel levissimam (ut sic dicam) haeresim silendo praeterirent, quin ei totis fidei viribus resisterent et scriptis ac disputationibus esse detestandam ac damnabilem demonstrarent. That is, "following the custom of the Fathers, who not once in their time, not in the slightest, refrained from silencing heresy (as I shall call it), but rather resisted it with all the strength of their faith, and showed it, through writings and arguments, to be detestable and damnable."
Yet while his interpretation of Islam was still basically negative, it did manage in “setting out a more reasoned approach to Islam…through using its own sources rather than those which were the products of the hyperactive imagination of some earlier Western Christian writers.” Although this alternative approach was not widely accepted or emulated by other Christian scholars of the Middle Ages, it did achieve some influence among a limited number of Church figures, including Roger Bacon.