on Trier, from the Catholic Encyclopedia
The beginnings of the see of Trier are obscure. 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. His immediate successors were St. Maximinus who sheltered the excommunicated St. Athanasius at Trier, and St. Paulinus, who was exiled to Phrygia on account of his opposition to Arianism.