1 To David himself; for understanding; by which it is understood that not by the merits of works, but by the grace of God, man is delivered, confessing his sins.
2. “Blessed are they whose unrighteousness is forgiven, and whose sins are covered”: and whose sins are buried in oblivion. “Blessed is the man to whom the Lord has not imputed sin, nor is there guile in his mouth”: nor has he in his mouth boastings of righteousness, when his conscience is full of sins.
3. “Because I kept silence, my bones waxed old:” because I made not with my mouth “confession unto salvation,” all firmness in me has grown old in infirmity. “Through my roaring all the day long”: when I was ungodly and a blasphemer, crying against God, as though defending and excusing my sins.
4. “Because day and night Your Hand was heavy upon me:” because, through the continual punishment of Your scourges, “I was turned in misery, while a thorn was fixed through me”: I was made miserable by knowing my misery, being pricked with an evil conscience.
5. “I acknowledged my sin, and my unrighteousness have I not hid:” that is, my unrighteousness have I not concealed. “I said, I will confess against myself my unrighteousness to the Lord:” I said, I will confess, not against God (as in my ungodly crying, when I kept silence), but against myself, my unrighteousness to the Lord. “And Thou forgavest the iniquity of my heart”; hearing the word of confession in the heart, before it was uttered with the voice.
6. “For this shall every one that is holy pray unto You in an acceptable time:” for this wickedness of heart shall every one that is righteous pray unto You. For not by their own merits will they be holy, but by that acceptable time, that is, at His coming, who redeemed us from sin. “Nevertheless in the flood of great waters they shall not come near him”: nevertheless, let none think, when the end has come suddenly, as in the days of Noah, that there remains a place of confession, whereby he may draw near unto God.
7. “You are my refuge from the pressures, which have compassed me about:” You are my refuge from the pressure of my sins, which has compassed my heart. “O Thou, my Rejoicing, deliver me from them that compass me about”: in You is my joy: deliver me from the sorrow which my sins bring upon me.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)