13 But let us see now another way in which this sentence may be understood. There is a certain iniquity, on the worker whereof it cannot be that God have mercy. You enquire, perchance, what that is? It is the defending of sins. When a man defends his sins, great iniquity he works: that thing he is defending which God hates. And see how perversely, how iniquitously. Whatever of good he has done, to himself he would have it to be ascribed; whatever of evil, to God. For in this manner men defend sins in the person of God, which is a worse sin....Therefore you defend your sin in such sort, that you lay blame on God. So the guilty is excused, so that the Judge may be charged. However on men working iniquity God has no pity at all.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)