20 “But He is Himself merciful, and will become propitious to their sins, and He will not destroy them. And He will abound to turn away His anger, and He will not kindle all his anger”. By these words many men promise to themselves impunity for their iniquity from the Divine Mercy, even if they shall have persevered in being such, as that generation is described, “crooked and embittering; which has not directed their heart, and the spirit thereof has not been trusted with God:” with whom it is not profitable to agree.
For if, to speak in their words, God will perchance not destroy no not even bad men, without doubt He will not destroy good men. Why then do we not rather choose that wherein there is no doubt? For they that lie to Him in their tongue, though their heart does hold some other thing, do think indeed, and will, even God to be a liar, when He does menace upon such men eternal punishment. But while they do not deceive Him with their lying, He does not deceive them with speaking the truth.
These words therefore of divine sayings, concerning which the crooked generation does cajole itself, let it not make crooked like its own heart: for even when it is made crooked, they continue right. For at first they may be understood according to that which is written in the Gospel, “that you may be like your Father who is in the Heavens, who makes His sun to rise upon good men and evil men, and rains upon just men and unjust men.” For who could not see, how great is the long-suffering of mercy with which He is sparing evil men?
But before the Judgment, He spared then that nation in such sort, that He kindled not all His anger, utterly to root it up and bring it to an end: which thing in His words and in the intercession for their sins of His servant Moses does evidently appear, where God says, “Let Me blot them out, and make you into a great nation:” he intercedes, being more ready to be blotted out for them than that they should be; knowing that he is doing this before One Merciful, who inasmuch as by no means He would blot out him, would even spare them for his sake. For let us see how greatly He spared, and does still spare....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)