The Christian Church has become synonymous with the Roman Empire by about 380 , but Theodosius finds his realm divided between rival orthodox and Arian Christian groups.

Propelled by political as well as religious motives, he energetically undertakes to bring about unity of faith within the empire.

Soon dominating his weak colleague, Theodosius enters the battle on the side of orthodoxy, becoming the first emperor who does not assume the title of pontifex maximus (supreme guardian of the old Roman cults).

His position has been improved by the fact that during 379 the followers of the Nicene Creed had gained ground, whereupon Theodosius, without consulting the ecclesiastical authorities, issues an edict on February 28, 380, prescribing a creed that is to be binding on all subjects.

Only persons who believe in the consubstantiality of God the Father, Son, and Holy Spirit are henceforth to be considered Catholic Christians, a designation that here appears for the first time in a document.

Although Theodosius’ belief in the Nicene Creed is apparently genuine, he submits to baptism in the fall of 380 only after a serious illness.