13 But the patience of good men with preparation of heart accepts the will of God: and glories in tribulations, saying that which follows: “Prepared is my heart, O God, I will sing and play”. What has he done to me? He has prepared a pit, my heart is prepared. He has prepared pit to deceive, shall I not prepare heart to suffer? He has prepared pit to oppress, shall I not prepare heart to endure? Therefore he shall fall into it, but I will sing and play. Hear the heart prepared in an Apostle, because he has imitated his Lord: “We glory,” he says, “in tribulations: because tribulation works patience: patience probation, probation hope, but hope makes not ashamed: because the love of God is shed abroad in our hearts through the Holy Spirit, which has been given to us.” He was in oppressions, in chains, in prisons, in stripes, in hunger and thirst, in cold and nakedness, in every wasting of toils and pains, and he was saying, “We glory in tribulations.” Whence, but that prepared was his heart? Therefore he was singing and playing.
14. “Rise up, my glory”. He that had fled from the face of Saul into a cavern, says, “Rise up, my glory:” glorified be Jesus after His Passion. “Rise up, psaltery and harp.” He calls upon what to rise? Two organs I see: but Body of Christ one I see, one flesh has risen again, and two organs have risen. The one organ then is the psaltery, the other the harp. Organs is the word used for all instruments of musicians. Not only is that called an organ, which is great, and blown into with bellows; but whatsoever is adapted to playing and is corporeal, whereof for an instrument the player makes use, is said to be an organ. But distinguished from one another are these organs....What therefore do these two organs figure to us? For Christ the Lord our God is waking up His psaltery and His harp; and He says, “I will rise up at the dawn.” I suppose that here ye now perceive the Lord rising. We have read thereof in the Gospel: see the hour of the Resurrection. How long through shadows was Christ being sought? He has shone, be He acknowledged; “at the dawn” He rose again. But what is psaltery? What is harp? Through His flesh two kinds of deeds the Lord has wrought, miracles and sufferings: miracles from above have been, sufferings from below have been. But those miracles which He did were divine; but through Body He did them, through flesh He did them. The flesh therefore working things divine, is the psaltery: the flesh suffering things human is the harp. Let the psaltery sound, let the blind be enlightened, let the deaf hear, let the paralytics be braced to strength, the lame walk, the sick rise up, the dead rise again; this is the sound of the Psaltery. Let there sound also the harp, let Him hunger, thirst, sleep, be held, scourged, derided, crucified, buried. When therefore you see in that Flesh certain things to have sounded from above, certain things from the lower part, one flesh has risen again, and in one flesh we acknowledge both psaltery and harp. And these two kinds of things done have fulfilled the Gospel, and it is preached in the nations: for both the miracles and the sufferings of the Lord are preached.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)