In 374, the bishop of Milan kicked the bucket, and the Arians had a hissy fit over his successor, because they were deathly afraid that a Catholic would be given the see. Well, Ambrose was governor of Aemilia-Liguria, and went to the church where the election was held to prevent a violent outcome. In a turn of events that only God could bring about, the people began to acclaim him as bishop. The Arians knew him to be Trinitarian, but thought his charity towards them in the matter made him acceptable. Only a catechumen (remember, many chose to be baptized later in life in the ancient days of the Church) without formal theological studies, he refused the post, and fled to the home of a friend. The Emperor Gratian wrote a letter to the friend praising Rome's appointment of worthy men to ecclesiastical positions, so his host gave Ambrose up. He was baptized, ordained, and consecrated bishop all within the span of a week (By the way, it's still theoretically possible for the latter two to be occur that quickly if the episcopal see in question is that of the Diocese of Rome...baptized Catholic male is the only explicitly defined requirement to be elected; the canons and traditions are somewhat conflicted on the issue.)
|His relics along with those of Sts Protasius and Gervasius|
On several occasions, the empress Justina demanded that Ambrose give up churches so that the Arians could use them. Each time he refused, and stood against imperial troops and even continued to say Mass on one occasion. A contrast with the East is noted, since he always won in his conflicts with the empire (John Chrysostom fought much weaker emperors and managed to lose...). He threatened to excommunicate Theodosius, not for Arianism- he was a staunch adherent to the Nicene Creed- but for massacring 7,000 Greeks after the Roman governor of Thessalonica was murdered. Also, Ambrose convinced them to further restrict pagan practices.
|The altar and ciborium of the Basilica|
A rite of the Latin Church, known as the Ambrosian Rite, is named in his honor. Though unlikely to be the exact rite of his time, the principal distinguishing practices probably had developed by then according to St Augustine's Confessions. (Remember, he was baptized by Ambrose.) This seems to stem from his occupation of the Portian basilica in a dispute with the empress as she wanted it for the Arians. Ambrose had the people sing Psalms hymns through the night, and introduced antiphonal singing into the Mass and office in order to rebut the Arian usage of the liturgy. This spread into the Roman Rite over time but its usage remained specific to Milan.