[3] As early as the second half of the 3rd century St. Piat evangelized Tournai; some sources name him as the first bishop, but this remains unsubstantiated.

He confided the Diocese of Arras and Cambrai to St. Vaast (Vedastus), and founded the See of Tournai (c. 500), appointing as its titular Eleutherius.

[4] It was probably its status of royal city which secured Tournai's early rise, only to lose its position as capital upon the departure of the Merovingian court.

Tournai lost its privileges and was relegated to level of the neighbouring dioceses, such as Boulogne and Therouanne, Arras and Cambrai.

[4] Notable bishops are: St. Eleutherius (beginning of 6th century); St. Achar (626/27 – 1 March 637/38); St. Eloi (641–660); Simon de Vermandois (1121–1146); Walter de Marvis (1219–1251), the great founder of schools and hospitals; Etienne of Tournai (1192–1203), godfather of Louis VII of France and minister to the queen; Andrea Chini Malpiglia (1334–42), cardinal and papal legate; Guillaume Fillastre (1460–1473), chancellor of the Golden Fleece; Michel de Warenghien (1283–1291), a very erudite doctor; Michel d'Esne (1597–1614), the author of several works.

Raphael de Mercatellis (1487–1507), illegitimate son of Philip the Good and noted bibliophile, was auxiliary bishop of Tournai.

The chapter's requirement to appoint only nobility and scholars, as set forth by the old régime, tended to attract the highly born and educated.

In 1559, to support the war against Protestantism, King Philip II of Spain obtained from Paul IV the foundation of a series of new dioceses.

The French Revolution created the Department of Jemappes, which in 1815 became the Province of Hainaut, whose borders coincided with those of the Diocese of Tournai, after a concordat between the plenipotentiaries of Pius VI and the consular government of the republic.