Ambrose Bishop of Milan

His commemoration is held on December 7.

A synopsis of his life from earlychurch.org's website:

Born the son of Aurelius Ambrosius, the imperial viceroy of Gaul,[1] Ambrose followed in his father's footsteps and became governor of Aemilia-Liguria (Northern Italy) in AD 370. He might well have remained in a political career had not dramatic events overtaken him. Ambrose had his official residence in Milan, where he was a catechumen in the church. When the bishop of the city, a pro-Arian named Auxentius died, the election of a new bishop was hotly contested by the Arian and Nicene parties within the church. The contest became so violent that Governor Ambrose was summoned to the church where the election was taking place because of reports of a riot. The story goes on to tell how someone shouted out "Bishop Ambrose!" a cry that was taken up and resulted in his being elected bishop.[2] Being only a catechumen he was hurriedly rushed through the various church orders and was ordained only eight days later, on 7th December 373.[3] Whatever the reasons for his election Ambrose proved his determination to succeed in his new position. His training and particularly his knowledge of Greek[4] set him in good stead. He devoted himself to prayer and to the study of both the Scriptures and pagan literature, rapidly becoming an accomplished preacher and writer.[5] His (sic) was able to read the works of the eastern writers in their original language and he made use of Philo, Origen, Athanasius, Didymus the Blind, Cyril of Jerusalem, Basil of Caesarea, Hippolytus of Rome and the Neoplatonist, Plotinus.[6] Ambrose is credited as being chiefly responsible for the final defeat of Arianism within the Western church.[7] Familiarity with Greek also enabled Ambrose to introduce allegorical interpretation to the western church.[8] He was deeply influenced by Philo and Origen, seeing in Scripture three levels of meaning: the literal, the moral and the allegorical, but also making use of typology.[9] His hermeneutic was of great help to Augustine, who refers to Ambrose in his Confessions, in removing his objections to the Old Testament Scriptures.[10]

From Ambrose's treatise Of the Christian Faith (Book I.VIII.54)

It is plain, therefore, that the Son is not unlike the Father, and so we may confess the more readily that He is also eternal, seeing the He Who is like the Eternal must needs be eternal. But if we say that the Father is eternal, and yet deny this of the Son, we say that the Son is unlike the Father, for the temporal differeth from the eternal. The Prophet proclaims Him eternal; the Testaments, Old and New alike, are full of witness to the Son's eternity.

And from the same treatise (Book IV.XII.169)

...For after this premised, He proceeded immediately: "I am the true Vine, and My Father is the Husbandman," that you might know that the Father is greater in so far as He dresses and tends our Lord's flesh, as the husbandman dresses and tends his vines. Father, our Lord's flesh is that which could increase in stature with age, and be wounded through suffering, to the end that the whole human race might rest guarded from the pestilent heat of the pleasures of this world, under the shadow of the Cross whereon Its limbs are spread.

Ambrose also served as a Catechist for Augustine of Hippo and his Confessions (Book V.13) tell of their first encounter.

In Milan I found your devoted servant the bishop Ambrose, who was known throughout the world as a man whom there were few to equal in goodness.  At that time his gifted tongue never tired of dispensing the richness of your corn, the joy of our oil, and the sober intoxication of your wine (references to Psalms 4.8, 44.8, 80.17. "Sober intoxication" comes from Ambrose's hymn "Splendor paternae gloriae."). Unknown to me, it was you who led me to him, so that I might knowingly be led by him to you. This man of God received me like a father and, as bishop, told me know glad he was that I had come. My heart warmed to him, not at first as a teacher of the truth, which I had quite despaired of finding in your Church, but simply as a man who showed me kindness. I listened attentively when he preached to the people, though not with proper intention....(Chapter 14) and while I was all ears to seize upon his eloquence, I also began to sense the truth of what he said, though only gradually. First of all it struck me that it was, after all, possible to vindicate his arguments. I began to believe the Catholic faith, which I had thought impossible to defend...

The Collect for St. Ambrose

O God, you gave your servant Ambrose grace eloquently to proclaim your righteousness in the great congregation, and fearlessly to bear reproach for the honor of your Name: Mercifully grant to all bishops and pastors such excellence in preaching and faithfulness in ministering your Word, that your people may be partakers with them of the glory that shall be revealed: through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, now and for ever. Amen.