1....The Title of this Psalm thus speaks: “At the end, corrupt not.” What is, “corrupt not?” That which You have promised, perform. But when? “At the end.” To this then let the mind's eye be directed, “unto the end.” Let all the things which have occurred in the way be passed over, in order that we may attain to the end. Let proud men exult because of present felicity, let them swell with honours, glitter in gold, overflow with domestics, be encircled with the services of clients: these things pass away, they pass away like a shadow. When that end shall have come, when all who now hope in the Lord are to rejoice, then to them shall come sorrow without end. When the meek shall have received that which the proud deride, then the vapouring of the proud shall be turned into mourning. Then shall there be that voice which we know in the Book of Wisdom: for they shall say at that time when they see the glory of the Saints, who, when they were in humiliation, endured them; who, when they were exalted, consented not— at that time then they shall say, “These are they whom sometime we have had in derision.” Where they also say, “What has pride profited us, and the boasting of riches has bestowed upon us what?” All things have passed away like a shadow. Because on things corruptible they relied, their hope shall be corrupted: but our own hope at that time shall be substance. For in order that the promise of God may remain whole and sure and certain towards us, we have said out of a heart of faith, “at the end corrupt not.” Fear not, therefore, lest any mighty man should corrupt the promises of God. He does not corrupt, because He is truthful; He has no one more mighty by whom His promise may be corrupted: let us be then sure concerning the promises of God; and let us sing now from the place where the Psalm begins.
2. “We will confess to You, O Lord, we will confess to You, and will invoke Your name”. Do not invoke, before thou confess: confess, and invoke. For Him whom you are invoking, unto yourself you call. For what is it to invoke, but unto yourself to call? If He is invoked by you, that is, if He is called to you, unto whom does He draw near? To a proud man He draws not near. High indeed He is, one lifted up attains not unto Him. In order that we may reach all exalted objects, we raise ourselves, and if we are not able to reach them, we look for some appliances or ladders, in order that being exalted we may reach exalted objects: contrariwise God is both high, and by the lowly He is reached. It is written, “Nigh is the Lord to them that have bruised the heart.” The bruising of the heart is Godliness, humility. He that bruises himself is angry with himself. Let him make himself angry in order that he may make Him merciful; let him make himself judge, in order that he may make Him Advocate. Therefore God does come when invoked. Unto whom does He come? To the proud man He comes not....Take heed therefore what ye do: for if He knows, He is not unobservant. It is better therefore that He be unobservant than known. For what is that same being unobservant, but not knowing? What is, not to know? Not to animadvert. For even as the act of one avenging animadversion is wont to be spoken of. Here one praying that He be unobservant: “Turn away Your face from my sins.” What then will you do if He shall have turned away His face from you? A grievous thing it is, and to be feared, lest He forsake you. Again, if He turn not away His face, He animadverts. God knows this thing, God can do this thing, namely, both turn away face from one sinning, and not turn away from one confessing....Confess therefore and invoke. For by confessing you purge the Temple, into which He may come, when invoked. Confess and invoke. May He turn away face from your sins, not turn away from you: turn away face from that which you have wrought, not turn away from that which He has Himself wrought. For you, as man, He has Himself wrought, your sins you have yourself wrought....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)