St. Ambrose

Ambrose of Milan, canonized as Saint Ambrose, was a Roman theologian, statesman, preacher, and Bishop of Milan from 374 until his death. Trained in rhetoric and law, he served as Roman civil governor of Aemilia-Liguria at Milan before being unexpectedly chosen as bishop by popular acclamation in 374. His preaching, ecclesiastical activity, and writings made him one of the most influential Christian figures of the 4th century.
As bishop, Ambrose became a prominent defender of Christian orthodoxy, particularly against Arianism, while also attempting to mediate the conflict between the emperors Theodosius I and Magnus Maximus. He opposed the execution of Priscillianist heretics and later excommunicated Theodosius following a massacre, maintaining that "an emperor is within the church, not over it." He was also a strong advocate of monasticism and asceticism, and his theological activity helped interpret and transmit Eastern Christian theology to the Latin West.
Ambrose left a substantial body of writings.