The prophecy of Isaiah is called, and correctly so, the gospel of the Old Testament. Look at chapter 53 and you will see why. For there, more clearly than anywhere else in the Old Testament, we find the good news of Christ and His cross, and our redemption from sin and guilt.
Consider also that Mount Zion was one of the hills in Jerusalem. The king's throne was set there. In Isaiah 1:27 we read, "Zion shall be redeemed with judgment, and her converts with righteousness." Because Zion was the place where the king's throne was, it represented that whole kingdom of Israel over which the king ruled. And the king and the nation represented Christ and His kingdom.
The truth then in this verse is that the church is redeemed with judgment, and her converts with righteousness. The comfort here is that the church with all its true members is redeemed, that is, the church is bought fully out of its guilt, and is freed from the punishment it in itself deserves.
The cost of our redemption is the precious blood of Christ, Who suffered our punishment, and brought to God the works of obedience which we failed to bring to any degree. Christ, the King of Zion, bought salvation for us. In Isaiah's day the people of God could say that Zion shall be redeemed. Today we can and must say that Zion is redeemed, is made righteous before God. By His righteousness Christ made us righteous.
Never, then, boast of your own works. Instead, confess your sins. Never claim to be in that kingdom, the church, because of what you did. Do that and you are showing that you do not deserve to be in it. Do not minimize the glory of Christ, the King, by boasting of what you did for Him. Instead thank Him for the work of redeeming you. And thank God for having sent Him to do all this for His church.
Read: Isaiah 1:1-27 .
O. Palmer Robinson: "Israel was unique among people of the world in that God himself appointed a priesthood for the nation—with accompanying laws of sacrifice and ritual—which carefully defined the right way to approach God. The laws of the Levitical priesthood, along with its festival days and sacrifices, contained touches of glamour and glory. Colorful robes, impressive ceremonies, feasts, washings, the waving of recently harvested grain, and the chanting of divine benedictions all contributed to the allurement of the priestly order of the old covenant. So it should not be surprising that throughout the centuries the Jewish people have had difficulty relinquishing these treasured ceremonies. They all contributed to making them feel right and good in the presence of God. Furthermore, when the new covenant came along with its minimal ritual, it seemed as though something significant had been lost" (The Israel of God, pp. 53-54).