Saint Ambrose, the Bishop that excommunicated an Emperor – a quick view

Ambrose had been Bishop of Milan from 374–397, whose multiple roles as chief opponent of Arianism, advisor to western emperors, eloquent preacher, arranger of hymns and psalms, developer of the cult of relics, and erudite author, only hint at the enormous influence he exerted over church and state in theWest, and at the originality
of his life and career.

Born into the highest social class, he was a respected provincial governor when the entire city acclaimed him bishop. He went from unbaptized Christian to bishop in eight days. More than any previous bishop, he played an important role in
the politics of his day, as illustrated by his excommunication of Theodosios I after that emperor massacred thousands of civilians in Thessalonike in reprisal for the murder of some German troops garrisoned there. Theodosios was forced to yield to the penance Ambrose imposed on him, and, thus, yield to Ambrose’s insistence that the church is independent from the state, and even superior to it in questions of faith and morals. He exerted a great influence on Augustine, who writes (in the Confessions, Book Five, Chapter 13) of Ambrose’s kindness, generosity, erudition, and eloquence.

(Source: «Historical Dictionary of Byzantium», by John H. Rosser)