25 “He scatters mist like ashes.” “He scatters,” says the Psalmist, “mist like ashes.” Who? He “who gives snow like wool.” For whom He predestined, He calls to repentance; for “whom He predestined, them He also called.” But “ashes” are connected with repentance. Hear Him calling to repentance, when He upbraided certain cities, saying, “Woe unto you, Chorazin! Woe unto you, Bethsaida! For if the mighty works which have been done in you had been done in Tyre and Sidon, they had long ago repented in dust and ashes.” Therefore, “He scatters mist like ashes.”
What is, “He scatters mist like ashes”? When a man is called to learn about God, and it is said to him, “Receive the truth;” he begins to wish to receive the Truth, but is not able; he sees that He is under a sort of darkness, which before he saw not....Wander not in the mist, follow in faith. But forasmuch as you endeavour to see and art not able, repent of your sins, for mist is scattered like ashes. Repent you now of having been obstinate against God, repent of having followed your own evil ways.
You have come into this state where it is difficult for you to see the vision of bliss, and the mist will be healthful to you, which God scatters like ashes. Thou yourself art as yet a mist, but like ashes. For they that are penitent, as yet roll themselves in ashes, my brethren, testifying, as it were, that they are like it, saying unto God, “I am ashes.” For a certain Scripture says, “I have despised myself, and wasted away, I have reckoned myself earth and ashes.” This is the humility of the penitent. When Abraham speaks to his God, and wishes the burning of Sodom to be disclosed to him, he says, “I am but earth and ashes.” How has this humility ever been found in great and holy men!
Source: The Enarrations, or Expositions, on the Psalms (New Advent)