Theodosius, the future emperor, had been born in 347 in Cauca in the province of Gallaecia in northwestern Spain.

His father was to become the general Flavius Theodosius; his mother's name is unknown.

His grandparents, like his parents, were probably already Christians.

Growing up in Spain, Theodosius had not received an extensive education but is intellectually open-minded and has acquired a special interest in the study of history.

While on his father's staff, he has participated in his campaigns against the Picts and Scots in Britain in 368-369, the Alamanni in Gaul in 370, and the Sarmatians in the Balkans in 372-373.

As a military commander in Moesia, a Roman province on the lower Danube, he defeats the Sarmatians in 374.