6 What of others? For all are not so conquered as to be converted and believe: many continue in obstinacy, many preserve in heart the spirit of going before, and if they exert it not, yet they labour with it, and finding opportunity bring it forth. Of such, what follows? “Let them be as dust before the wind”. “Not so are the ungodly, not so; but as the dust which the wind drives away from the face of the earth.” The wind is temptation; the dust are the ungodly. When temptation comes, the dust is raised, it neither stands nor resists.
“Let them be as dust before the wind, and let the Angel of the Lord trouble them.” “Let their way be darkness and slipping”. A horrible way! Darkness alone who fears not? A slippery way alone who avoids not? In a dark and slippery way how shall you go? Where set foot? These two ills are the great punishments of men: darkness, ignorance; a slippery way, luxury. “And let the Angel of the Lord persecute them;” that they be not able to stand. For any one in a dark and slippery way, when he sees that if he move his foot he will fall, and there is no light before his feet, haply resolves to wait until light come; but here is the Angel of the Lord persecuting them.
These things he predicted would come upon them, not as though he wished them to happen. Although the Prophet in the Spirit of God so speaks these things, even as God does the same, with sure judgment, with a judgment good, righteous, holy, tranquil; not moved with wrath, not with bitter jealousy, not with desire of wreaking enmities, but of punishing wickedness with righteousness; nevertheless, it is a prophecy.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)