From the time of Diocletian reorganization of the divisions of the empire, Trier was the capital of Belgica Prima, the chief city of Gaul, and frequently the residence of the emperors. There were Christians among its population as early as the second century, and there was probably as early as the third century a bishop at Trier, which is the oldest episcopal see in Germany. The first clearly authenticated bishop is Agricius who took part in the Council of Arles in 314.
An 11th-century tradition states that he had been a priest of Antioch, and that he was moved to the See of Trier by Pope Silvester I at the request of the Empress Helena. He was present at the Council of Arles in 314, and signed the acts immediately after the presiding bishop of that diocese. This act meant that, at least in the fourth century, Trier laid claim to the primacy of Gaul and Germany, a claim his successor Saint Maximin reinforced. This story seems to have developed in order to promote the primacy of Trier over other sees in Gaul and Germany.
According to tradition, the "Heiliger Rock" is said to be the robe which Jesus wore before His crucifixion and for which, according to the Gospel of St. John, the Roman soldiers cast lots. Part of this robe is said to have been found by Helena, mother of Constantine the Great, who gave it to Agricius.
Saint Athanasius, who came as an exile to Trier in 335 or 336, speaks of the large numbers of faithful whom he found there and the number of churches in the course of being built. The famous relics of Trier, the "Heiliger Rock", the Nail of the True Cross, and the body of Matthias the Apostle are said to have been brought there by Agricius.