23 “She answered Him in the way of His strength”....The preceding words show, that either “His praise,” or “Jerusalem,” answered: for it was said, “And His praise in Jerusalem; in the nations gathering together in one, and the kingdoms, that they may serve the Lord. Respondit ei.” We cannot say, “the kingdoms answered,” for he would have said responderunt. Respondit ei. We cannot say, “the nations answered,” for he would have said, responderunt (in the plural).
Since then it is Respondit ei, in the singular, we look for the singular number above, and find that the words, “His praise,” and “Jerusalem,” are the only words in which we find it. But since it is doubtful, whether it be “His praise,” or “Jerusalem,” let us expound it each way. How did “His praise” answer Him? When they who are called by Him thank Him. For He calls, we answer; not by our voice, but by our faith; not by our tongue, but by our life....From His elect and holy men, Jerusalem also answers Him.
For Jerusalem also was called: and the first Jerusalem refused to hear, and it was said unto her, “Behold, your house shall be left unto the desolate.”...But that Jerusalem, of whom it was written, “Sing, O barren, you that did not bear,” “She has answered Him.” What means, “She has answered Him”? She despises Him not when He called. He sent rain, She gave fruit.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)