Constantine had summoned to Trier the Christian theologian and polemicist Lactantius, to be the tutor of Crispus, who on March 1, 317, is given he title of caesar and made titular ruler of Gaul; Constantine's second son receives the title of Caesar at the same time.

The Diocese of Gaul (Latin: Dioecesis Galliarum, "diocese of the Gaul [province]s"), established in about 314, encompasses northern and eastern Gaul; that is, modern France north and east of the Loire, including the Low Countries and modern Germany west of the Rhine.

The diocese comprises the following provinces: Gallia Lugdunensis I, Gallia Lugdunensis II, Gallia Lugdunensis III, Gallia Lugdunensis IV (Senonia), Belgica I, Belgica II, Germania I, Germania II, Alpes Poenninae et Graiae and Maxima Sequanorum.