Today is the feast of St. Ambrose (340-397), one of the early Doctors of the Church. Ambrose was a governor in northern Italy, and became bishop of Milan on December 7 after he made peace with the Arians (who denied the deity of Christ) and Athanasians (who affirmed the Trinity) when they were fighting over who would be the next bishop. Through his subsequent preaching, most of that district was converted to the Athanasian position. Ambrose was also the person who led Augustine to his conversion (as told in his Confessions).
Ambrose was further famous for his hymnwriting abilities. His hymns are some of the earliest that are extant. When Arian soldiers wished to enter his church to worship, he famously barricaded the door with his congregation and gave them Trinitarian hymns to sing. The soldiers felt unable to harm hymn-singing people, and so were prevented from entering. Many (if not all) of Ambrose's hymns include a last verse praising the Trinity, a helpful feature for someone battling Arianism.
One of Ambrose's hymns that is still sung currently is an Advent hymn, Veni, Redemptor gentium. It has been translated a few times, but the one I know is the translation of John Mason Neale, whom you might remember from before. It is usually sung to the tune PUER NOBIS NASCITUR, a 15th century tune harmonized by Michael Praetorius. Note its very strong emphasis on the twofold nature of Christ (especially verses four through six).
Come, Thou Redeemer of the earth,
And manifest Thy virgin birth:
Let every age adoring fall;
Such birth befits the God of all.
Begotten of no human will,
But of the Spirit, Thou art still
The Word of God in flesh arrayed,
The promised Fruit to man displayed.
The virgin womb that burden gained
With virgin honor all unstained;
The banners there of virtue glow;
God in His temple dwells below.
Forth from His chamber goeth He,
That royal home of purity,
A giant in twofold substance one,
Rejoicing now His course to run.
From God the Father He proceeds,
To God the Father back He speeds;
His course He runs to death and hell,
Returning on God’s throne to dwell.
O equal to the Father, Thou!
Gird on Thy fleshly mantle now;
The weakness of our mortal state
With deathless might invigorate.
Thy cradle here shall glitter bright,
And darkness breathe a newer light,
Where endless faith shall shine serene,
And twilight never intervene.
All laud to God the Father be,
All praise, eternal Son, to Thee;
All glory, as is ever meet,
To God the Holy Paraclete.