5 Guard the mouth of your heart from evil, and you will be innocent: the tongue of your body will be innocent, your hands will be innocent; even your feet will be innocent, your eyes, your ears, will be innocent; all your members will serve under righteousness, because a righteous commander has your heart. “Then shall they say among the heathen, the Lord has done great things for them.”
6. “Yea, the Lord has done great things for us already, whereof we rejoice”. Consider, my brethren, if Sion does not at present say this among the heathen, throughout the whole world; consider if men are not running unto the Church. In the whole world our redemption is received; Amen is answered. The dwellers in Jerusalem, therefore, captive, destined to return, pilgrims, sighing for their country, speak thus among the heathen. What do they say? “The Lord has done great things for us, whereof we rejoice.” Have they done anything for themselves? They have done ill with themselves, for they have sold themselves under sin. The Redeemer came, and did the good things for them.
7. “Turn our captivity, O Lord, as the torrents in the south”. Consider, my brethren, what this means....As torrents are turned in the south, so turn our captivity. In a certain passage Scripture says, in admonishing us concerning good works, “Your sins also shall melt away, even as the ice in fair warm weather.” Our sins therefore bound us. How? As the cold binds the water that it run not. Bound with the frost of our sins, we have frozen. But the south wind is a warm wind: when the south wind blows, the ice melts, and the torrents are filled. Now winter streams are called torrents; for filled with sudden rains they run with great force. We had therefore become frozen in captivity; our sins bound us: the south wind the Holy Spirit has blown: our sins are forgiven us, we are released from the frost of iniquity; as the ice in fair weather, our sins are melted. Let us run unto our country, as the torrents in the south....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)