He was born to humble parents and his given name is not known; his father was Lorenzo Antonini, of Canepina, near Viterbo, and his mother, Maria del Testa. He entered the Order of St. Augustine in June 1488 at which time he was given the name Giles. After a course of studies at priories of the Order in Ameria, Padua, Istria, Florence and Rome, where he studied philosophy. He was later made a doctor of theology. In 1506 became Vicar General of his Order. Upon the death of the Prior General, and, under the patronage of Pope Julius II, he was confirmed by election as his successor at three successive General Chapters of the Order: in 1507, 1511 and 1515.