Saint Ambrose (340 to 397 A.D.) was Bishop of Milan from 374 to 397 and an important early Catholic Church Father. He was also one of the most illustrious Doctors of the Church, and fitly chosen, together with St. Augustine, St. John Chrysostom, and St. Athanasius, to uphold the venerable Chair of the Prince of the Apostles in the tribune of St. Peter's at Rome.
Saint Ambrose was a lawyer before he became a priest and later a bishop. He attacked imperial moral standards and strengthened the position of the Church in the Roman Empire through his preaching and writing. Saint Augustine was one of his famous converts.
In 390 A.D., emperor Theodosius responded vindictively to sedition in Thessalonica by massacring 7000 people. When he turned up at Ambrose's church in Milan, Ambrose refused him entry until he had repented of his crime. Ambrose's reasoning was that the same Lord made both the emperor and the ordinary people. Theodosius subsequently did repent of his actions. Ambrose also got Theodosius to pass a new law that any sentences of death or proscription were to be stayed for 30 days, after which they were to be reviewed. In taking the actions that he did, Ambrose established that under Christianity, nobody, not even the emperor of Rome was above the law, and that the law was based on God's standards.
During his lifetime, Ambrose called several councils and worked tirelessly against the Arian heresy.
Some of his writings
- De Spiritu Sancto (On the Holy Ghost)
- De mysteriis (On the Mysteries)
- De incarnationis Dominicae sacramento (On the Sacrament of the Incarnation of the Lord)
- Homiletic commentaries on the Old Testament
- De excessu fratris Satyri (funeral orations)
- Ethical works: De bono mortis (Death as a Good); De fuga saeculi (Flight From the World); De paenitentia (On Repentance); De paradiso (On Paradise); De sacramentis (On the Sacraments); De viduis (On Widows); De virginibus (On Virgins); De virginitate (On Virginity); Exhortatio virginitatis (Exhortation to Virginity); De sacramento regenerationis sive de philosophia (On the Sacrament of Rebirth, or, On Philosophy [fragments]) 
- The New American Desk Encyclopedia, Penguin Group, 1989