Saint Ruelin (or Saint Rivelin) succeeded Saint Tudwal as Bishop of Tréguier in the sixth century.

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Ruelin was a student of the Breton monk Tudwal who had established a hermitage on an island off the coast of North Wales.

Around 540, he immigrated with Tudwal to Armorica. Ruelin founded a hermitage, later a monastery, at present-day Châteauneuf-du-Faou. When Tudwal was dying, the priests asked him to select a successor, and he named Ruelin. His election was contested by Saint Pergat, canon and archpriest of Lexobie. who won a part of the clergy and the people to his cause. To remedy this schism, a synod was assembled at Lexobie, where the most skillful and learned ecclesiastics of the diocese were convoked to resolve the dispute.^[1] It took an appearance from Tudwal for Pergat to retire, ending his days in penance at Pouldouran.^[2]