O Emmanuel, Rex et legifer noster, exspectatio gentium, et Salvator earum: veni ad salvandum nos Domine Deus noster.

O Emmanuel, God with us, our King and lawgiver, the expected of the nations and their Savior: come to save us, O Lord our God.

Isaias 7:14
Therefore the Lord himself shall give you a sign. Behold a virgin shall conceive, and bear a son and his name shall be called Emmanuel.

From my publication, Praying the Mass during Advent by Dom Gueranger:

O Emmanuel! King of Peace! thou enterest today the city of thy predilection, the city in which thou hast placed thy Temple,—Jerusalem. A few years hence, and the same city will give thee thy Cross and thy Sepulcher: nay, the day will come, on which thou wilt set up thy Judgment-seat within sight of her walls. But, today, thou enterest the city of David and Solomon unnoticed and unknown. It lies on thy road to Bethlehem. Thy Blessed Mother and Joseph, her Spouse, would not lose the opportunity of visiting the Temple, there to offer to the Lord their prayers and adoration. They enter; and then, for the frst time, is accomplished the prophecy of Aggeus, that great shall be the glory of this late House more than of the first; for this second Temple has now standing within it an Ark of the Covenant more precious than was that which Moses built; and within this Ark, which is Mary, there is contained the God, whose presence makes her the holiest of sanctuaries. The Lawgiver himself is in this blessed Ark, and not merely, as in that of old, the tablet of stone on which the Law was graven. The visit paid, our living Ark descends the steps of the Temple, and sets out once more for Bethlehem, where other prophecies are to be fulflled. We adore thee, O Emmanuel! in this thy journey, and we reverence the fdelity wherewith thou fulfllest all that the prophets have written of thee, for thou wouldst give to thy people the certainty of thy being the Messiah, by showing them, that all the marks, whereby he was to be known, are to be found in thee. And now, the hour is near; all is ready for thy Birth; come, then, and save us; come, that thou mayest not only be called our Emmanuel, but our Jesus, that is, He that saves us.