2 The Prophet sings to Him of the future, and uses words as it were of past time: he speaks of things future as if already done, because with God that which is future has already taken place....“Lord, You have been favourable unto Your land”; as if He had already done so. “You have turned away the captivity of Jacob.” His ancient people of Jacob, the people of Israel, born of Abraham's seed, in the promise to become one day the heir of God. That was indeed a real people, to whom the Old Testament was given; but in the Old Testament the New was figured: that was the figure, this the truth expressed.
In that figure, by a kind of foretelling of the future, there was given to that people a certain land of promise, in a region where the people of the Jews abode; where also is the city of Jerusalem, whose name we have all heard of. When this people had received possession of this land, they suffered many troubles from their neighbouring enemies who surrounded them: and when they sinned against their God, they were given into captivity, not for destruction, but for discipline; their Father not condemning, but scourging them.
And after being seized on, they were set free, and many times were both made captives, and set free; and they are now in captivity, and that for a great sin, even because they crucified their Lord. What then are we to understand them to mean by the words, “You have turned away the captivity of Jacob”?...This Psalm has prophesied in song. “You have turned away the captivity of Jacob.” To whom did it speak? To Christ; for it said, “for the end, for the sons of Core:” for He has turned away the captivity of Jacob.
Hear Paul himself confessing: “O wretched man that I am, who shall deliver me from the body of this death?” He asked who it should be, and straightway it occurred to him, “The grace of God through Jesus Christ our Lord.” Of this grace of God the Prophet speaks to our Lord Jesus Christ, “You have turned away the captivity of Jacob.” Attend to the captivity of Jacob, attend, and see that it is this: You have turned away our captivity, not by setting us free from the barbarians, with whom we had not met, but by setting us free from bad works, from our sins, by which Satan held sway over us. For if any one has been set free from his sins, the prince of sinners has not whence he may hold sway over him.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)