Guillaume Erart, a native of the diocese of Langres, was a doctor of theology; be is not to be confused with Guillaume Evrard. He was a master of arts, a bachelor of theology, and became rector of the University on February 26, 1421.

He was procureur of the "nation" of France in 1426 and was en rapport with Jean Graverent, the Inquisitor, on the subject of heretics who had appealed to the Pope. He received his degree as licentiate and then as master of theology in 1428. He taught at Paris from September, 1428, at the same time as Pierre de Dyerée, Pierre Le Mire, Jean Gravestain, and Guillaume Adelie. In December, 1430, he pleaded before the Parlement against Geoffroy le Normant, declaring that he had been ordained "master of grammarians of the College de Navarre."

In 1429 Guillaume Erart had been sent to Champagne by the King of England, along with Pierre Cauchon; Geoffroy protested that it was not his duty to teach children "and he ought to employ himself in preaching"; he declared besides that he had more than thirty livres income, was cure in Normandy with eighty francs income, and that he was a canon of Laon and canon and sacristan of Langres. Erart made allusion to a journey that he had made in Germany, to Bâle. We find him at Paris in September, 1431, among the master regents, and on January 25, 1432, he presided at Paris over the examination for the licentiate, when Thomas de Courcelles received first rank. Nicolas Loiseleur was his first pupil at Paris in 14311432. And Guillaume Bonnel, Abbot of Cormeilles, dean of the Faculty of Law, took action against him for "at the last licentiate examination (1432) he opened the list of licentiates which the masters had given him."

In 1433 he is called the vice-chancellor of Nôtre Dame. In August in the name of the University he made before Parlement at Paris a "grief compliante" on the subject of the royal ordinance on the repurchase of rentes of the churches and colleges of Paris. Guillaume Erart is mentioned for the last time among the master regents in September, 1433. Henceforth he was in Normandy.

He entered the Chapter of Rouen on July 17, 1432. Later he is to be found in Paris where he went to "further the liberties of the Church." Named archdeacon of Grand Caux in 1433, he fulfilled in turn the trusts of chancellor, precentor and vicar-general. On February 7, 1434, he was named professor of theology and received a canonicate at Nôtre Dame in Paris by recommendation of the Bishop of Paris, Jacques du Châtelier, a worldly prelate who was elevated to the rank of bishop by the favor of Bedford and Philippe le Bon of Burgundy. He was the executor of the will of Hugues des Orges, Archbishop of Rouen in 1434, and he disputed with the canons of Nôtre Dame the subject of the succession to office by the wealthy Alespée and the latter alienated him from the Chapter. He appealed to the Pope.

A friend of Louis de Luxembourg, Guillaume Erart went to England to swear fealty in his name to Henry VI for the perpetual administration of the church of Ely. The King of England charged him to go to Arras in company with Raoul Roussel and Jean de Rinel in 1435 to treat for peace, and he responded very dryly to the fine and pacific discourse of Thomas de Courcelles. On November 12, 1436, Guillaume Erart presided at an assembly of prelates "for a certain need touching public affairs and the coming of certain English lords" to Normandy.

In 1437 he was named chaplain to the King and received an annual income of twenty pounds sterling for services rendered to the Crown. Guillaume Erart from that time on lived in England, and we note that the canons of Rouen charged him to reclaim the legacies made to their church by Henry V and Bedford. Named dean of the cathedral in place of the late Gilles Deschamps, Guillaume Erart did not take possession of this dignity, which, however, he accepted, and which shows all the hope the canons placed in his influence at the English court.

He died in England in 1439, leaving great sums to the cathedral of Rouen and to the college of the community, and an enameled silver chalice to the Chapter. He bequeathed likewise a legacy Of 40 livres to the University of Paris. The executor of his will was the rigorous Pasquier de Vaux, Bishop of Evreux.

English at heart, like his patron Louis de Luxembourg, very active and unscrupulous, he appears to us as one of the most impassioned judges of Jeanne d'Arc, whom he condemned violently on the day of her abjuration.