The Southern Picts, for whom Ninian is held to be the apostle, are the Picts south of the mountains known as the Mounth, which cross Scotland north of the Firths of Clyde and Forth. That they had once been Christian is known from a 5th-century mention of them by Saint Patrick in his "Letter to Coroticus", where he refers to them as 'apostate Picts'. Patrick could not have been referring to the Northern Picts who were converted by Saint Columba in the 6th century because they were not yet Christian, and thus could not be called 'apostate'. Northumbria had established a bishopric among the Southern Picts at Abercorn in 681, under Bishop Trumwine. This effort was abandoned shortly after the Picts defeated the Northumbrians at the Battle of Dun Nechtain in 685.