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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 "O Antiphon" is a cry to the "Key of David"
Latin: O Clavis David, et sceptrum domus Israel;qui aperis, et nemo claudit;claudis, et nemo aperit:veni, et educ vinctum de domo carceris,sedentem in tenebris, et umbra mortis.
English: O Key of David, and scepter of the House of Israel, who open and no one closes, who close and no one opens, come and lead forth from the house of bondage, the captive city in darkness and the shadow of death
This title of "Key of David" is first and foremost a reference to Isaiah 22:22, which notes
I will place the key of the House of David on his shoulder; what he opens, no one will shut, what he shuts, no one will open.
We see this title of Christ brought into the New Testament as well. In the Book of Revelation we read in 3:7 -
“To the angel of the church in Philadelphia, write this: “The holy one, the true, who holds the key of David, who opens and no one shall close, who closes and no one shall open says this:"
Knowing that Christ is the Key of David, we then can understand more fully the words of Matthew 16:19 where Jesus tells Peter as the head of the Apostles and the Church:
"I will give you the keys to the kingdom of heaven. Whatever you bind on earth shall be bound in heaven; and whatever you loose on earth shall be loosed in heaven.”
It is from Matthew 16:19 that the Church derives its understanding of the Sacrament of Confession - the idea that the Apostles, firstly Peter, were given the authority to either bound up or free people from their sins. People argue all the time "why did Christ set it up that way?" The answer is that He didn't have to set it up that way, He could have said "stay at home and confess your sins on your couch" but He didn't say that. While we many not understand Christ's reasons for everything, as St. Thomas Aquinas notes, what we can be sure of is that if Christ set something up, we can be sure it is the best way.
Thus we see that until he returns, the way in which, as the antiphon says, we are lead out of the "house of bondage, the captive city and the shadow of death" is through the Sacrament of Confession.