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These "O Antiphons" are ancient pieces of chant sung on the days from December 17th through December 23rd. Each day a different Old Testament title for the messiah is beseeched to come. As the world cried out in longing for the Messiah before Christ, so we still cry out for his return.
Today's Antiphon is sung to "Radix Jesse"
Latin: O Radix Jesse, qui stas in signum populorum, super quem continebunt reges os suum, quem Gentes deprecabuntur: veni ad liberandum nos, jam noli tardare.
English: O root of Jesse, who stand as a sign for the peoples, whom kings will kneel with silence, whom nations will entreat with prayer, come to set us free, delay no longer.
The kings will seek out the Lord (the "wise men" are the first to do it) will seek out the Lord because he is the answer to the question that is every human heart (as John Paul II would say, paraphrasing Gaudium et Spes paragraph 22).
This title of the Messiah as the "Root of Jesse" clearly references the prophecy found in Isaiah 11:1-13
When we read in Isaiah about the "stump of Jesse" it is an image not unlike the "family tree" image we use today. The family tree of Jesse, who was the Father of the first king of Israel, King David, had run its course and was no longer capable of bearing fruit. The kings had corrupted and the fruit of Jesse's tree, through the generations, had withered away to a barren stump.
But as anyone who has spent time in the woods can tell you, sometime where a stump is dead, a new plant can spring forth from the stump. It is quite cool to see a new plant growing on a dead stump. That is exactly what Isaiah is prophesying - the dead tree of Israel's kings will, in the Messiah, see a new plant shoot forth from the dead stump, a new king, a king that will be sought out by all the kingdoms of the Earth.
The prophecy as we read it in Isaiah:
But a shoot shall sprout from the stump of Jesse, and from his roots a bud shall blossom. The spirit of the LORD shall rest upon him. A spirit of wisdom and of understanding, A spirit of counsel and of strength, a spirit of knowledge and of fear of the LORD, and his delight shall be the fear of the LORD.
Not by appearance shall he judge, nor by hearsay shall he decide, But he shall judge the poor with justice, and decide fairly for the land’s afflicted. He shall strike the ruthless with the rod of his mouth,
and with the breath of his lips he shall slay the wicked. Justice shall be the band around his waist,
and faithfulness a belt upon his hips. Then the wolf shall be a guest of the lamb, and the leopard shall lie down with the young goat; The calf and the young lion shall browse together, with a little child to guide them. The cow and the bear shall graze, together their young shall lie down; the lion shall eat hay like the ox. The baby shall play by the viper’s den, and the child lay his hand on the adder’s lair.
They shall not harm or destroy on all my holy mountain; for the earth shall be filled with knowledge of the LORD, as water covers the sea.
On that day, The root of Jesse, set up as a signal for the peoples— Him the nations will seek out; his dwelling shall be glorious. On that day, The Lord shall again take it in hand to reclaim the remnant of his people that is left from Assyria and Egypt, Pathros, Ethiopia, and Elam, Shinar, Hamath, and the isles of the sea. He shall raise a signal to the nations and gather the outcasts of Israel; The dispersed of Judah he shall assemble from the four corners of the earth.