Hauntingly Beautiful Ancient Hymn by St. Ambrose for Advent "Come, Redeemer of the Nations" mentioned by St. Augustine

St. Ambrose, bishop of Milan, in 374, found a liturgy in use which tradition associates with St. Barnabas. He composed some of the very first hymns used by the Church. This particular hymn "Veni, redemptor gentium" in Latin was translated to "O come, Redeemer of the Nations." St. Augustine who was baptized by St. Ambrose mentions this hymn. It was later translated to the German, "Nun komm, der Heiden Heiland" which is a Lutheran chorale of 1524 with words written by Martin Luther, based on "Veni redemptor gentium".
Full Lyrics of the Advent hymn for the Office of the Readings for the octave before Christmas.

Veni, redemptor gentium, ostende partum Virginis; miretur omne saeculum: talis decet partus Deum.	O Come, Redeemer of the earth, and manifest thy virgin-birth. Let every age in wonder fall: such birth befits the God of all.
Non ex virili semine, sed mystico spiramine Verbum Dei factum est caro fructusque ventris floruit.	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.
Alvus tumescit Virginis, claustrum pudoris permanet, vexilla virtutum micant, versatur in templo Deus.	The Virgin's womb that burden gained, its virgin honor still unstained. The banners there of virtue glow; God in his temple dwells below.
Procedat e thalamo suo, pudoris aula regia, geminae gigas substantiae alacris ut currat viam.	Proceeding from His chamber free that royal home of purity a giant in twofold substance one, rejoicing now His course to run.
Aequalis aeterno Patri, carnis tropaeo cingere, infirma nostri corporis virtute firmans perpeti.	O equal to the Father, Thou! gird on Thy fleshly mantle now; the weakness of our mortal state with deathless might invigorate.
Praesepe iam fulget tuum lumenque nox spirat novum, quod nulla nox interpolet fideque iugi luceat.	Thy cradle here shall glitter bright, and darkness breathe a newer light where endless faith shall shine serene and twilight never intervene.
Sit, Christe, rex piissime, tibi Patrique gloria cum Spiritu Paraclito, in sempiterna saecula. Amen.	All praise, eternal Son, to Thee, whose advent sets Thy people free, whom, with the Father, we adore, and Holy Ghost, for evermore. Amen.

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Latin from the Liturgia Horarum. Translation by J. M. Neale (1818-1866).