Peter Thomas was born into a very poor family (his father, a farm head, was a serf) in southern Périgord around 1305 (thought to be today’s Lebreil, a section of Salles-de-Belves, about forty kilometers to the southwest of Sarlat (Dordogne), a traditional center of devotion to the saint).
Upon the death of his brother, Peter Thomas, left his parents and his younger sister while still a young man so as to lighten the burdens on his family. He went on to the nearby small town of Monpazier, forty-five kilometers from Bergerac. Here he attended school for about three years, living on alms and teaching younger pupils. He led the same type of life at Agen until the age of twenty, when he returned to Monpazier.
The prior Of the Carmelite convent of Lectoure took note of him and had him teach for a year in that school. Then the prior of Condom or, more probably, that Of Bergerac brought him to his own convent and gave him the Carmelite habit. Peter Thomas made his profession of religious vows at Bergerac and taught there for two years.
As a lector of logic at Agen, he also studied philosophy there and, after three years, was ordained a priest. He was helped in his dire poverty by the intervention of Our Lady, and went to teach logic in the Carmelite convent of Bordeaux for one year, then philosophy at the Carmelite house at Albi, and then again in Agen.
After a stay of three years at Paris to further his studies, while preaching during a procession at Cahors held to overcome a tremendous drought, he caused a «miraculous rain to fall.
Three years later he returned to Paris to continue his four-year course and gain the baccalaureate in theology. On his return to his own province he was elected procurator general by the chapter of May 15, 1345, and was sent to the Roman curia, which was the pontifical court at Avignon.
Despite the fact that physically he was not well-gifted (his Father General was ashamed to present him to the cardinals), he was noticed by his fellow-countryman, the cardinal of Périgord, Elias Talleyrand, who had him named apostolic preacher. The cardinal also intervened to permit him to finish his studies at Paris and to be declared a master in sacred theology towards his third year rather than after the five years prescribed by the university. He returned to Avignon in approximately 1351 and successfully resumed his office of apostolic preacher.
At the death of Pope Clement VI, he accompanied the corpse to the Chaise-Dieu, preaching at all the twelve stops along the way (April, 1353).
From that time on the whole life of Peter Thomas was dedicated to the fulfillment of delicate missions entrusted to him by the Holy See, for peace among Christian princes, for the defense of the rights of the Church before the most powerful monarchs of the age, for the union of the Orthodox Byzantine–Slavs with the Roman Church, for the anti-Muslim crusade and the liberation of the Holy Land.