Even if Emilia, Romagna and the territory around Venice were Christianized and had bishops before Piedmont, for example, their conversion does not go back beyond the end of the second century.

[5] On 1 May 1818, in the bull "De Salute", Pope Pius VII removed the diocese of Adria from subjection as a suffragan to the Archdiocese of Ravenna, and assigned it to the Patriarchate of Venice.

[citation needed] In a decree of the Second Vatican Council, it was recommended that dioceses be reorganized to take into account modern developments.

[11] A project begun on orders from Pope John XXIII, and continued under his successors, was intended to reduce the number of dioceses in Italy and to rationalize their borders in terms of modern population changes and shortages of clergy.

[12] At the same time, it seemed appropriate "for a more appropriate looking out for the good of souls" that the Basilica of S. Stephen Pope and Martyr be granted the dignity of being a co-cathedral.

The cathedral was administered and staffed by a corporation called the Chapter, consisting of an archpriest and seven canons; there were also twelve priests who carried out various religious functions.