1....How man had come into captivity, let us ask the Apostle Paul....For he says: “For we know that the Law is spiritual, but I am carnal, sold under sin.” Behold whence we became captives; because we were sold under sin. Who sold us? We ourselves, who consented to the seducer. We could sell ourselves; we could not redeem ourselves. We sold ourselves by consent of sin, we are redeemed in the faith of righteousness. For innocent blood was given for us, that we might be redeemed. Whatsoever blood he shed in persecuting the righteous, what kind of blood did he shed? Righteous men's blood, indeed, he shed; they were Prophets, righteous men, our fathers, and Martyrs. Whose blood he shed, yet all coming of the offspring of sin. One blood he shed of Him who was not justified, but born righteous: by shedding that blood, he lost those whom he held. For they for whom innocent blood was given were redeemed, and, turned back from their captivity, they sing this Psalm.
2. “When the Lord turned back the captivity of Sion, we became as those that are comforted”. He meant by this to say, we became joyful. When? “When the Lord turned back the captivity of Sion.” What is Sion? Jerusalem, the same is also the eternal Sion. How is Sion eternal, how is Sion captive? In angels eternal, in men captive. For not all the citizens of that city are captives, but those who are away from thence, they are captives. Man was a citizen of Jerusalem, but sold under sin he became a pilgrim. Of his progeny was born the human race, and the captivity of Sion filled all lands. And how is this captivity of Sion a shadow of that Jerusalem? The shadow of that Sion, which was granted to the Jews, in an image, in a figure, was in captivity in Babylonia, and after seventy years that people turned back to its own city....But when all time is past, then we return to our country, as after seventy years that people returned from the Babylonish captivity, for Babylon is this world; since Babylon is interpreted “confusion.”...So then this whole life of human affairs is confusion, which belongs not unto God. In this confusion, in this Babylonish land, Sion is held captive. But “the Lord has turned back the captivity of Sion.” “And we became,” he says, “as those that are comforted.” That is, we rejoiced as receiving consolation. Consolation is not save for the unhappy, consolation is not save for them that groan, that mourn. Wherefore, “as those that are comforted,” except because we are still mourning? We mourn for our present lot, we are comforted in hope: when the present is passed by, of our mourning will come everlasting joy, when there will be no need of consolation, because we shall be wounded with no distress. But wherefore says he “as” those that are comforted, and says not comforted? This word “as,” is not always put for likeness: when we say “As,” it sometimes refers to the actual case, sometimes to likeness: here it is with reference to the actual case....Walk therefore in Christ, and sing rejoicing, sing as one that is comforted; because He went before you who has commanded you to follow Him.
3. “Then was our mouth filled with joy, and our tongue with exultation”. That mouth, brethren, which we have in our body, how is it “filled with joy”? It uses not to be “filled,” save with meat, or drink, or some such thing put into the mouth. Sometimes our mouth is filled; and it is more that we say to your holiness, when we have our mouth full, we cannot speak. But we have a mouth within, that is, in the heart, whence whatsoever proceeds, if it is evil, defiles us, if it is good, cleanses us. For concerning this very mouth ye heard when the Gospel was read. For the Jews reproached the Lord, because His disciples ate with unwashen hands. They reproached who had cleanness without; and within were full of stains. They reproached, whose righteousness was only in the eyes of men. But the Lord sought our inward cleanness, which if we have, the outside must needs be clean also. “Cleanse,” He says, “the inside,” and “the outside shall be clean also.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)