[4] Another Epictetus, Bishop of Centumcellæ towards the middle of the fourth century, was an Arian and a counsellor of Emperor Constantius.

Pope Leo IV, needing a secure port on the Tyrrhenian Sea, built a new fortified city 13 km (8 mi) away from the ruins of Centumcellae, which he dedicated in 853.

[7] By 1092 Civitavecchia e Toscanella was united with the diocese of Viterbo in the person of Bishop Riccardus, who died in that year or earlier.

[12] In 1854 the union with Santa Rufina was severed, and Civitavecchia was united with the diocese of Corneto (Tarquinia).

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

[15] 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.

There was to be only one episcopal curia, one seminary, one ecclesiastical tribunal; and all the clergy were to be incardinated in the diocese of Civitavecchia-Tarquinia.