
Have pity on me, O God,
according to your great mercy.
According to the magnitude of your compassions
blot out my iniquity.
Wash me thoroughly from my lawlessness,
and cleanse me from my sin.
For I know my lawlessness,
and my sin is always before me.
Against you alone did I sin,
and I did the evil deed before you,
so that you would be vindicated in your words
and prevail when you judge.
For behold, in evils I was conceived,
and in sins my mother conceived me.
For behold, you loved truth.
You showed to me the unknown things
and the secret things of your wisdom.
Purify me with hyssop,
and I will be cleansed.
Wash me,
and I will be made more white than snow.
Cause me to hear great joy and merriment.
The bones, having been humbled, will rejoice exceedingly.
Turn your face from my sins,
and blot out all my lawless acts.
Create in me a clean heart, O God,
and renew a right spirit in my inward parts.
Do not cast me from your face,
and do not remove your Holy Spirit from me.
Restore to me the great joy of your salvation,
and strengthen me with a guiding spirit.
I will teach the lawless ones your ways,
and the ungodly ones will return to you.
(Psalm 50:3–15 LXX [Psalm 51:1–13])
I ask you, how great was the blessed man’s humility in acknowledging his fault when he showed such constancy in making satisfaction after he was pardoned? That sin of adultery is shown to have been foreign and uncharacteristic, since it was lamented with such concentration of mind. The sudden confession of the thief attracts us; we rejoice that Peter’s tears were quickly in evidence; the short-lived humility of the publican we find charming. But David with his more prolonged attempt to wipe away his sins afforded all men a chance to absolve themselves. He ensured that his tears, running down the faces of people who came after him, are dried with no lapse of time. Let us note also the prophet’s humility. An inner voice terrified that prince’s heart and he directed his anger at himself, for he knew that the rebuke by which he was blamed was just. Then that ruler over huge nations became his own harshest torturer, demanding from himself a punishment which he could scarcely have borne at another’s command. It is the common practice to excuse one’s own sins with specious explanations, but this most powerful king elected instead to accuse himself in the sight of all. He whose judgment the people had been accustomed to fear pronounced himself guilty.…
The prophet is aware that the Lord’s love is such that He does not cease to be just as well, and he appropriately introduced justice into his plea so that his request would be more readily heard through the incorporation of justice. He knows that sin must be punished, but says that the Lord must spare him because he confirms from his own mouth that he is condemned. As Solomon puts it: The just man is his own accuser at the start of his speech. So let us observe that he says: I know. The sins which we commit with knowledge are graver; those which we do in ignorance are not. Alternatively, all can know their sins, but the only persons known to acknowledge them are those seen to condemn them by personally cursing them. Perfect repentance lies in avoiding future sins and lamenting those of the past. Initially after his sin, when the prophet questioned him, David replied that the man who coveted the poor man’s ewe not belonging to him was worthy of death; at that time David did not believe that his sin should be lamented. But now as he prostrates himself and utters suppliant groans, he regrets that the sins stand before him like some shadow of a ghost. He added: Always, because he continually sees the sin, even when he closes his eyes. This regular contemplation of his sins reveals the perseverance of devoted supplication. Next time we look at such things with the mind’s eye, we regret having done them. The Lord said in the previous psalm: I will reprove you and set your iniquity before your face. This is what the most holy man did to himself when he said: And my sin is always before me. He was right to seek forgiveness, for he seemed to have observed the shape of the judgment to come. This figure is called procatalepsis or anticipation, for David fears the most grisly appearance of his sins as though already on trial at the coming judgment.
Cassiodorus, Explanation of the Psalms 50