4 And what is it they are now to hear? “My mouth shall speak of wisdom, and the meditation of my heart understanding”. And this repetition is perhaps made, lest perchance if he had said only “my mouth,” you should suppose that one spoke to you who had understanding but in his lips. For many have understanding in their lips, but have not in their heart, of whom the Scripture says, “This people honours me with their lips, but their heart is far from me.” What says he then who speaks to you? When he has said, “My mouth shall speak of wisdom,” in order that you may know that what is poured forth from the mouth flows from the bottom of the heart, he has added, “And the meditation of my heart of understanding.”
5. “I will incline mine ear to the parable, I will show my proposition upon the harp”....And why “to a parable”? Because “now we see through a glass darkly,” as says the Apostle; “while we are at home in the body, we are absent from the Lord.” For our vision is not yet that face to face, where there are no longer parables, where there no longer are riddles and comparisons. Whatever now we understand we behold through riddles. A riddle is a dark parable which it is hard to understand. Howsoever a man may cultivate his heart and apply himself to apprehend mysteries, so long as we see through the corruption of this flesh, we see but in part....But as He was seen by those who believed, and by those who crucified Him, when He was judged; so will He be seen, when He shall have begun to be judge, both by those whom He shall condemn, and by those whom He shall crown. But that vision of divinity, which He has promised to them that love Him, when He says, “He that loves Me shall be loved of My Father, and he that loves Me keeps My commandments, and I will love him, and will manifest Myself to him:” this the ungodly shall not see. This manifestation is in a certain way familiar: He keeps it for His own, He will not show it to the ungodly. Of what sort is the vision itself? Of what sort is Christ? Equal to the Father. Of what sort is Christ? “In the beginning was the Word, and the Word was with God, and the Word was God.” For this vision we sigh now, and groan so long as we sojourn here; to this vision we shall be brought home at the last, this vision now we see but darkly. If then we see now darkly, let us “incline our ear to the parable,” and then let us “show our proposition upon the harp:” let us hear what we say, do what we enjoin.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)