27 Thus then we see “the snow, the mist, the crystal:” it is good that He blow and thaw them. For if He blow not, if He Himself thaw not the hardness of this ice, “in the face of His cold who shall stand?” He abandons a sinner, behold, He calls him not; behold, He opens not his perception; behold, He pours not in grace; let the man thaw himself, if he can, from the ice of folly. He cannot. Wherefore can he not? “In the face of His cold who shall stand?” Behold him then growing harder, and saying, “O wretched man that I am!
Who shall deliver me from the body of this death?” Behold, I am growing cold, behold, I am growing hard, what heat shall thaw me that I may run? “Who shall deliver me from the body of this death?....In the face of His cold who shall stand?” And who shall free himself, if God abandon him? Who is it that frees? “The grace of God, through Jesus Christ our Lord.” Are we then to despair? God forbid. For it goes on, “He shall send out His Word, and melt them”. Let not then the snow despair, nor the mist, nor the crystal.
For of the snow, as of wool, a garment is being made. That mist finds safety in repentance: for, “whom He predestinated, them He also called.” But even though they be the very hardest among the predestinated, though they have been for a long time hardening, and have become crystal, they will not be hard to the mercy of God. “He shall send out His Word, and melt them.” What is “melt”? Understand not “melt” in an ill sense: it means, He shall liquefy, He shall thaw them. For they are hard through pride.
Rightly is pride called also dulness: for whatever is dull, is also cold....Despair not even of the crystal. Hear a saying of the crystal. “Who before was a blasphemer, and a persecutor, and injurious.” But wherefore does God melt the crystal? That the snow despair not of itself. For he says, “For this cause I obtained mercy, that in me first Jesus Christ might show forth all long-suffering, for a pattern to them that hereafter should believe in Him unto eternal life.” God then calls unto the Gentiles, “Be melted, O crystal; come, you snows.”
“His Spirit shall blow, and the waters shall flow.” Lo, the “crystal” and the “snows” are melted, they turn into water, “let them that thirst, come and drink.” Saul, hard as crystal, persecuted Stephen unto death; Paul, now in the living water, calls the Gentiles to the Fount....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)