O Adonai, et Dux domus Israel,
qui Moysi in igne flammae rubi apparuisti,
et ei in Sina legem dedisti:
veni ad redimendum nos in brachio extento.

O Adonai, and leader of the House of Israel,
who appeared to Moses in the fire of the burning bush
and gave him the law on Sinai:
Come and redeem us with an outstretched arm.

But with righteousness he shall judge the poor, and decide with equity for the meek of the earth; and he shall smite the earth with the rod of his mouth, and with the breath of his lips he shall slay the wicked. Righteousness shall be the girdle of his waist, and faithfulness the girdle of his loins. Isaiah 11:4-5

For the LORD is our judge, the LORD is our ruler, the LORD is our king; he will save us. Isaiah 33:22

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During the evening prayer of the Liturgy of Hours, which is also known as Vespers, the Church prays the beautiful prayer of Mary known as the Magnificat. Preceding the Magnificat, there is an antiphon which is simply a short verse which connects the prayer to the particular feast day or liturgical season. On the last seven days of Advent, from December 17th through December 23rd, the antiphon for Vespers is one of the so-called "O Antiphons". It is related to a particular title of Christ's, and the antiphon ends with a plea for the Messiah to come. Click here to read more about the O Antiphons.