A local tradition that dates back to the beginning of the twelfth century tells us that Taurinus, fifth bishop of Eauze (Elusa), abandoned his episcopal city, which was destroyed by the Vandals, and transferred his see to Auch.

He was the recipient, along with his three suffragan bishops Involatus of Comminges, Wainard of Couserans, and Garston of Tarbes (Bigorre), of a letter of Pope John VIII, in which the Pope complained that their parishioners were a people heavy with iniquity; that they were marrying without regard to church rules or public decency (including incest); that people were appropriating church goods for their own private use; and that priest, clerics, and laity were failing to obey their bishops.

In one, the Pope complained that unworthy persons were being intruded, sometimes by secular persons, into parishes in the dioceses of the ecclesiastical Province of Auch, for which tasks they were unfit and had been rejected by their bishops; the archbishop was authorized to warn the bishops, to take an appeal, and, if they do not obey, to issue canonical censures and remove the offending clergy from their churches.

Another mandate complained that reports had reached the Pope that, in the Province of Auch, monks, canons and other religious had left their cloisters and were visiting the courts of local magnates in neglect of their vows and monastic discipline, and their superiors; the archbishop was ordered to give them a warning and then, if they disobeyed, to suspend them from their offices and benefices.

In a third mandate, the Pope advised the archbishop that in his Province there were clergy who were accumulating dignities and other benefices, including archdeaconries and cathedral dignities; the archbishop was granted special powers to advise the persons affected of their transgression, and invite them to choose one of the benefices and surrender the rest, and, if they do not cooperate, to apply ecclesiastical censures.

Finally, in yet another letter, Innocent III reminds the archbishop of Auch that he was well advised by him and by his fellow bishops that Gascony was overrun by heretics, and he encourages and authorizes the archbishop to pursue them until they are cleared from his Province, by whatever effective measures he can muster, and suspending the right of appeal; and if necessary he may use the civil authorities (principes) and the people, and to coerce the heretics by the use of the material sword (si necesse fuerit, per principes et populum eosdem facias virtute materialis gladii coerceri).

He then pointed out that the diocese of Auch had fallen on bad times during Bernard's administration, or rather maladministration, and that it might be appropriate for him to consider a spontaneous resignation.

The commission was authorized to determine the truth of the charges, and if appropriate to depose the archbishop and select some other suitable person to succeed him.

[7] The current cathedral of the diocese of Auch, dedicated to the Blessed Virgin Mary, is a Gothic structure with a neo-classical Renaissance façade.

Georges Goyau describes the building as "imposing in spite of this incongruity", and praises its fifteenth-century windows as the most beautiful in France.

[12] The Council of Trent, in its 23rd session, on 15 July 1563, published a decree requiring all dioceses to have and maintain a seminary for youth studying for the priesthood.

Cardinal Luigi d'Este (died 1586) did not carry out the decree, due to non-residence, but he allocated funds in his last will and testament, which his sister, Antoinette d' Este, the Duchesse de Nemours, was to use for the founding of a seminary in Auch.

Archbishop La Tour du Pin-Montauban of Auch refused to take the oath to the Civil Constitution of the Clergy, and therefore his see was declared by the Legislative Assembly to be vacant.

The electors of Gers chose in his place the dean of the faculty at Toulouse, Canon Paul-Benoit Barthe, who was also president of the Amis de la Constitution.

He was arrested on suspicion in July 1793 and sent to Paris, where he had to appear before the Committee of Public Safety, but he was acquitted and allowed to return to Auch.

Then he began to restore the old Ancien Régime dioceses, or most of them, though not with the same boundaries as before the Revolution, but instead taking into account the new political structure of France with its departments instead of provinces.

The diocese of Auch was not one of those revived by Pope Pius VII in his bull Qui Christi Domini of 29 November 1801.