7 But I would not that you should seek without yourselves, how the Jordan was turned back, I would not ye should augur anything evil. For the Lord chides those who have “turned” their “back” unto Him, “and not their face.” And whoever forsakes the source of his being, and turns away from his Creator; as a river into the sea, he glides into the bitter wickedness of this world. It is therefore good for him that he turn back, and that God whom he had set behind his back, may be before his face as he returns; and that the sea of this world, which he had set before his face, when he was gliding on towards it, may become behind him; and that he may so forget what is behind him, that he may “reach forward to what is before him;” which is profitable for him when once converted....
8. “Tremble, thou earth, at the presence of the Lord, at the presence of the God of Jacob”. What means, “at the presence of the Lord,” save at the presence of Him who said, “Lo, I am with you always, even unto the end of the world.” For the earth trembled; but because it had remained slothful, it was made to tremble, so that it might be more firmly fixed at the presence of the Lord.
9. “Who turned the hard rock into standing waters, and the flint stone into springing wells”. For He melted Himself, and what may be called His hardness to water those who believe in Him, that He might in them become “a fountain of water gushing forth unto everlasting life;” because formerly, when He was not known, He seemed hard. Hence they who said, “This is an hard saying, who can bear it?” were confounded, and waited not until He should flow and stream upon them when the Scriptures were revealed. The rock, that hardness, was turned into pools of water, that stone into fountains of waters, when on His resurrection, “He expounded unto them, commencing with Moses and all the prophets, how Christ ought to suffer thus;” and sent the Holy Ghost, of whom He said, “If any man thirst, let him come unto Me, and drink.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)