From the LCMS website:
Born in Trier in A.D. 340, Ambrose was one of the four great Latin Doctors of the Church (with Augustine, Jerome, and Gregory the Great). He was a prolific author of hymns, the most common of which is Veni, Redemptor gentium (“Savior of the Nations, Come”). His name is also associated with Ambrosian Chant, the style of chanting the ancient liturgy that took hold in the province of Milan. While serving as a civil governor, Ambrose sought to bring peace among Christians in Milan who were divided into quarreling factions. When a new bishop was to be elected in 374, Ambrose addressed the crowd, and someone cried out, “Ambrose, bishop!” The entire gathering gave their support. This acclaim of Ambrose, a 34-year-old catechumen, led to his baptism on December 7, after which he was consecrated bishop of Milan. A strong defender of the faith, Ambrose convinced the Roman emperor Gratian in 379 to forbid the Arian heresy in the West. At Ambrose’s urging, Gratian’s successor, Theodosius, also publicly opposed Arianism. Ambrose died on Good Friday, April 4, 397. As a courageous doctor and musician he upheld the truth of God’s Word.