21 In the second place, that we may not seem to do violence to divine words, and lest in the place where there was said, “He will not destroy them,” we should say, “But hereafter He will destroy them:” concerning this very present Psalm let us turn to a very common phrase of the Scripture, whereby this question may be more diligently and more truly solved. Speaking of these same persons a little lower down, when He had made mention of the things which the Egyptians because of them had endured, He says,...“And He led them unto the mount of His sanctification, the mount which His right hand won. And He cast out from their face the nations, and by lot distributed to them the land in the cord of distribution.” If any one at these words should press a question upon us, and should say, How does he make mention of all these things as having been bestowed upon them, when the same persons were not led into the land of promise, as were delivered from Egypt, inasmuch as they were dead? What shall we reply but that they were spoken of, because they were the self-same people by means of a succession of sons?...
22. “And He remembered that they are flesh, a spirit going and not returning”. Therefore calling them and pitying them through His grace, He called them back Himself, because of themselves they could not return. For how does flesh return, “a spirit walking and not turning back,” while a weight of evil deserts does weigh it down unto the lowest and far places of evil, save through the election of grace?...For thus also is solved this no unimportant question, how it is written in the Proverbs, when the Scripture was speaking of the way of iniquity, “all they that walk in her shall not return.” For it has been so spoken as if all ungodly men were to be despaired of: but the Scripture did only commend grace; for of himself man is able to walk in that way, but is not able of himself to return, except when called back by grace.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)