5 “Neither is there any rest in my bones, from the face of my sin.” It is commonly enquired, of what person this is the speech; and some understand it to be Christ's, on account of some things which are here said of the Passion of Christ; to which we shall shortly come; and which we ourselves shall acknowledge to be spoken of His Passion. But how could He who had no sin, say, “There is no rest in my bones, from the face of my sin.”...For if we were to say that they are not the words of Christ, those words, “My God, My God, why have You forsaken Me?” will also not be the words of Christ.
For there too you have, “My God, My God, why have You forsaken Me?” “The words of mine offenses are far from my health.” Just as here you have, “from the face of my sins,” so there also you have, “the words of my offenses.” And if Christ is, for all that, without “sin,” and without “offenses,” we begin to think those words in the Psalm also not to be His. And it is exceedingly harsh and inconsistent that that Psalm should not relate to Christ, where we have His Passion as clearly laid open as if it were being read to us out of the Gospel.
For there we have, “They parted My garments among them, and cast lots upon My vesture.” Why should I mention that the first verse of that Psalm was pronounced by the Lord Himself while hanging on the Cross, with His own mouth, saying, “My God, My God, why have You forsaken Me?” What did He mean to be inferred from it, but that the whole of that Psalm relates to Him, seeing He Himself, the Head of His Body, pronounced it in His own Person? Now when it goes on to say, “the words of mine offenses,” it is beyond a doubt that they are the words of Christ.
Whence then come “the sins,” but from the Body, which is the Church? Because both the Head and the Body of Christ are speaking. Why do they speak as if one person only? Because “they two,” as He has said, “shall be one flesh.” “This” (says the Apostle) “is a great mystery; but I speak concerning Christ and the Church.”...For why should He not say, “my sins,” who said, “I was an hungred, and you gave Me no meat; I was thirsty, and you gave Me no drink; I was a stranger, and you took Me not in.
I was sick and in prison, and you visited Me not.” Assuredly the Lord was not in prison. Why should He not say this, to whom when it was said, “When saw we You a hungred, and thirsty, or in prison; and did not minister unto You?” He replied, that He spoke thus in the person of His Body. “Inasmuch as you did it not unto one of the least of Mine, you did it not unto Me.” Why should He not say, “from the face of my sins,” who said to Saul, “Saul, Saul, why do you persecute Me,” who, however, being in Heaven, now suffered from no persecutors?
But just as, in that passage, the Head spoke for the Body, so here too the Head speaks the words of the Body; while you hear at the same time the accents of the Head Itself also. Yet do not either, when you hear the voice of the Body, separate the Head from it; nor the Body, when you hear the voice of the Head: because “they are no more two, but one flesh.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)