5 But it was done; I would say these words to those that have not done the like, in order that they should watch to keep their uncorruptness, and that while they take heed how a great one has fallen, they that be small should fear. But if any that has already fallen hears these words, and that has in his conscience any evil thing; to the words of this Psalm let him advert; let him heed the greatness of the wound, but not despair of the majesty of the Physician. Sin with despair is certain death.
Let no one therefore say, If already any evil thing I have done, already I am to be condemned: God pardons not such evil things, why add I not sins to sins? I will enjoy this word in pleasure, in wantonness, in wicked cupidity: now hope of amendment having been lost, let me have even what I see, if I cannot have what I believe. This Psalm then, while it makes heedful those that have not believed, so does not will them that have fallen to be despaired of. Whoever you are that hast sinned, and hesitatest to exercise penitence for your sin, despairing of your salvation, hear David groaning.
To you Nathan the prophet has not been sent, David himself has been sent to you. Hear him crying, and with him cry: hear him groaning, and with him groan; hear him weeping, and mingle tears; hear him amended, and with him rejoice. If from you sin could not be excluded, be not hope of pardon excluded. There was sent to that man Nathan the prophet, observe the king's humility. He rejected not the words of him giving admonition, he said not, Darest thou speak to me, a king? An exalted king heard a prophet, let His humble people hear Christ.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)