Faustus was born between 405 and 410, and according to his contemporaries, Avitus of Vienne and Sidonius Apollinaris, in the island of Britain. Nothing is known about his early life or his education. He is thought by some to have been a lawyer but owing to the influence of his mother, famed for her sanctity, he abandoned secular pursuits as a young man and entered the monastery of Lérins. Here he was soon ordained to the priesthood and because of his extraordinary piety was chosen in 432 to be head of the monastery, in succession to Maximus who had become Bishop of Riez. His career as abbot lasted about twenty or twenty-five years during which he attained a high reputation for his wonderful gifts as an extempore preacher and for his stern asceticism. After the death of Maximus he became Bishop of Riez. This elevation did not make any change in his manner of life; he continued his ascetic practices, and frequently returned to the monastery of Lérins to renew his fervour. He was a zealous advocate of monasticism and established many monasteries in his diocese. In spite of his activity in the discharge of his duties as bishop, he participated in all the theological discussions of his time and became known as a stern opponent of Arianism in all its forms. For this, and as is said for his view, stated below, of the corporeity of the human soul, he incurred the enmity of Euric, King of the Visigoths, who had gained possession of a large portion of Southern Gaul, and was banished from his see. His exile lasted eight years, during which time he was aided by loyal friends. On the death of Euric he resumed his labours at the head of his diocese and continued there until his death between 490 and 495. His own diocesan flock considered him a saint and erected a basilica in his honour.
Throughout his life Faustus was an uncompromising adversary of Pelagius, whom he styled Pestifer 'plague bringer', and equally decided in his opposition to the doctrine of predestination which he styled "erroneous, blasphemous, heathen, fatalistic, and conducive to immorality". This doctrine in its most repulsive form had been expounded by a presbyter named Lucidus and was condemned by two synods, at Arles and Lyons (475). At the request of the bishops who composed these synods, and especially Leontius of Arles, Faustus wrote the Libri duo de Gratiâ Dei et humanae mentis libero arbitrio, in which he refuted not only the doctrines of the Predestinarians but also those of Pelagius (P.L., LVIII, 783). The work was marred, however, by its decided Semipelagianism, for several years was bitterly attacked, and was condemned by the Synod of Orange in 529 (Denzinger, Enchiridion, Freiburg, 1908, no. 174 sqq. - old no. 144; PL.L., XLV, 1785; Mansi, VIII, 712). Besides this error, Faustus maintained that the human soul is in a certain sense corporeal, God alone being a pure spirit. The opposition to Faustus was not fully developed in his lifetime and he died with a well-merited reputation for sanctity. Faustus wrote also: "Libri duo de Spiritu Sancto" (P.L., LXII, 9), wrongly ascribed to the Roman deacon Paschasius. His "Libellus parvus adversus Arianos et Macedonianos", mentioned by Genadius, seems to have perished.