1....If perchance things obscure demand the office of an interpreter, those things which are evident ought to require of me the office of a reader. The song here is of the Advent of the Lord and of our Saviour Jesus Christ, and of His vineyard. But the singer of the song is that Asaph, as far as does appear, enlightened and converted, by whose name ye know the synagogue to be signified. Lastly, the title of the Psalm is: “For the end in behalf of them that shall be changed;” that is, for the better. For Christ, the end of the Law, has come on purpose that He should change men for the better. And he adds, “a testimony to Asaph himself.” A good testimony of truth. Lastly, this testimony does confess both Christ and the vineyard; that is, Head and Body, King and people, Shepherd and flock, and the entire mystery of all Scriptures, Christ and the Church. But the title of the Psalm does conclude with, “for the Assyrians.” The Assyrians are interpreted, “men guiding.” Therefore it is no longer a generation which has not guided the heart thereof, but now a generation guiding. Therefore hear we what he says in this testimony.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)