The former French diocese of Thérouanne (Lat. Moriniensis) controlled a large part of the left bank of the river Scheldt during the Middle Ages. Territorially it was part of the county of Artois which belonged to the county of Flanders.
History
In the 7th century, probably around 639, Saint Audomar (Saint Omer) established the bishopric of Terwaan or Terenburg in Thérouanne. Thanks to that ecclesiastical control of some of the most prosperous cities north of the Alps, like Arras and Ypres, the bishopric was able to build a cathedral which was at the time the largest in France.
In 1099 the diocese of Thérouanne underwent a particularly wrenching experience. Their bishop, Gerard was denounced to Pope Urban II as a simoniac by the clergy of the diocese. He was unable to explain away the evidence and purge himself of the charge, and therefore he was compelled to resign and retire to the monastery of Saint-Eloi. The Canons of the Cathedral met and elected Erkembodus (Archambaud), Provost of the Cathedral of Saint-Omer as the new bishop. He declined the election. They then elected Aubert de Helcin, one of their own fellow canons, who was also Canon of Amiens. But then Lambert, the Abbot of Saint-Bertin held an election carried out by the clergy and laity of the diocese, which produced Jean de Warneton, who was a Canon of Lille and a religious of the monastery of Saint-Eloi. Abbot Lambert immediately set off for Rome, to have his choice ratified. On 25 April 1299, the third week after Easter, in a Council held in Rome, Pope Urban II quashed the election of Aubert de Helcin and upheld that of Jean de Warneton. Urban II wrote to the Metropolitan, Manasses of Reims, ordering him to have the clergy and people present Bishop-elect Jean to the Bishop of Arras for ordination to the priesthood. Then on 17 July 1099 Manasses himself consecrated Jean a bishop. Manasses was holding a council in Reims at the time, on instructions from Robert, Count of Flanders, to promote the Truce of God.
In 1303, there had been a fire in Thérouanne, as a result of the marauding and pillaging of some nobles and clerks during war in Flanders, which had led to murder and desecration of churches and cemeteries. On 8 March 1304 Pope Benedict IX authorized the Chapter of Thérouanne to use the money left by Bishop Henri de Murs for the establishment for a prebend in the Cathedral for the purpose of repairing the church of Nôtre-Dame which had been damaged in the fire. On the same day he authorized the Bishop to use the first year of income from vacant benefices and prebends for the repair of the Cathedral.
On 20 September 1346 the city of Thérouanne was besieged by the English and burned. Bishop Raimond Saquet was forced to flee and seek safety in Saint-Omer.
In 1553 Charles V besieged Thérouanne, then a French enclave in the Holy Roman Empire, in revenge for a defeat by the French at Metz. After he captured the city he ordered it to be razed to the ground and the roads to be broken up. In 1557, as a result of the war damage to its see, the diocese was abolished. About two decades later the diocese of Boulogne was created, bearing the name Thérouanne for a few years.
The disappearance of the former bishopric led to a reform of sees at the Council of Trent, and the bishopric of Thérouanne was split between the Diocese of Saint-Omer, the diocese of Boulogne and the Diocese of Ypres.