S. Teilo or Theliav was descended from a noble British stock. His father's name was Enlleu, son of Hydwn Dwn, and the place of his birth was Eccluis Cunnian, near Tenby. From his infancy he bent his neck beneath the gentle yoke of Christ, and, despising worldly vanities, was given to prayer and alms-deeds. He studied first under S. Dubricius, by whose assistance he attained great proficiency in the knowledge of the Scriptures; his next instructor was Paulinus, under whom he pursued the same study, and in whose school he was the associate of S. David. Under the patronage of Dubricius he opened a college, at Llandaff, which was called Bangor Deilo, or the great choir of Teilo; and his settlement at that place may serve to account for his appointment to fill the See of Llandaff upon the retirement of his patron to the Isle of Bardsey. The idea that he was made bishop of Llandaff at the time S. Dubricius was raised to the Archbishopric of Caerleon, is irreconcilable with chronology. The original diocese governed by S. Teilo, as ascertained by the absence of churches founded by S. David, was co-extensive with the ancient lordships of Glamorgan, containing the present rural deaneries of Croneath, Llandaff, and Newport. How long he continued to preside over this limited district is uncertain; but, in the reign of Maelgwn Cwynedd, the Yellow Plague desolated the Principality. Upon this occasion S. Teilo, with several others, retired to Cornwall, and thence into Brittany, where he was honourably received by S. Samson, Bishop of Dol. After he had remained seven years and as many months in Brittany, he returned, with several of his disciples, to his native country, and on his arrival was elected to the Archbishopric of Menevia, vacant by the death of Cynog. Like S. David, he, however, retained a predilection for the seat of his original bishopric, and, appointing Ismael to the situation of Bishop of Menevia, he removed the archbishopric to Llandaff. In order to maintain his title to the primacy undisturbed, he appears to have kept under his immediate government the whole of the diocese held before by S. David, with the exception of the part north of the river Tivy, which was henceforth attached to the diocese of Llanbadarn. There is a story told of his having made a pilgrimage, before all this, to the Holy Land, in company with S. David and S. Padarn, and of their having received episcopal consecration from the Patriarch of Jerusalem; but it is probable that this was a story, borrowed by Giraldus from one of the lost Triads, and was invented to establish the independence of the Welsh bishops of the See of Rome.