[1][2] The diocese, which adheres to the Roman Rite, was established on 18 April 1474 for political reasons, to transform the Marquisate of Montferrat into an ecclesiastic territory.

Casale Monferrato, the ancient Bodincomagus, is a city in the province of Alessandria, Piedmont (Italy), on the River Po, and has been a stronghold since the time of the Lombards.

Liutprand, King of the Lombards enlarged it,[3] and, in 936, the Emperor Otto II made it the chief town of a marquisate, giving it to the sons of Aleran, Duke of Saxony.

[5] The parish church of S. Evasius in Casale, and its college of Canons Regular of S. Augustine,[6] is known from a grant made to them on 15 August 988.

[13] Among its noteworthy bishops were: the Dominican Benedetto Erba (1570), most zealous for the Christian instruction of children and the introduction of the Tridentine reforms, in which he was associated with Archbishop Charles Borromeo of Milan; he was also the founder of the monti di pietà.

After a brief occupation by Austrian troops, the French, under the First Consul, Napoleon Bonaparte, reconquered the Piedmont, and established the Subalpine Republic, which lasted only from June to September 1802.

Thereafter the territory was annexed to the French Republic, and divided into the départements of Doire, Marengo, Pô, Sesia, Stura, and Tanaro.

[17] In the bull "Gravissimis Causis" of 1 June 1803, Pope Pius VII authorized the papal legate to First Consul Bonaparte, Cardinal Giovanni Battista Caprara, to suppress a number of dioceses in the ecclesiastical province of Piedmont, including Casale.

King Victor Emmanuel I of Sardinia invited Pope Pius VII to restore the good order of the Church in his kingdom, which had been disrupted by the French occupation.

On 17 July 1817, the pope issued the bull "Beati Petri", which began by establishing de novo the ten dioceses which had been suppressed under the French, and delimiting the extent of each in detail,[22] including Casale.