18 In praises of God, in confessions of sins, in hymns and in songs, in prayers, “There is a multitude of the sound of waters. The clouds have uttered a voice”. Thence that sound of waters, thence the troubling of the abysses, because “the clouds have uttered a voice.” What clouds? The preachers of the word of truth. What clouds? Those concerning which God does menace a certain vineyard, which instead of grape had brought forth thorns and He says, “I will command My clouds, that they rain no rain upon it.” In a word, the Apostles forsaking the Jews, went to the Gentiles: in preaching Christ among all nations, “the clouds have uttered a voice.” “For Your arrows have gone through.” Those same voices of the clouds He has again called arrows. For the words of the Evangelists were arrows. For these things are allegories. For properly neither an arrow is rain, nor rain is an arrow: but yet the word of God is both an arrow because it does smite; and rain because it does water. Let no one therefore any longer wonder at the troubling of the abysses, when “Your arrows have gone through.” What is, “have gone through”? They have not stopped in the ears, but they have pierced the heart. “The voice of Your thunder is in the wheel”. What is this? How are we to understand it? May the Lord give aid. When boys we were wont to imagine, whenever we heard thunderings from Heaven, that carriages were going forth as it were from the stables. For thunder does make a sort of rolling like carriages. Must we return to these boyish thoughts, in order to understand, “the voice of Your thunder is in the wheel,” as though God has certain carriages in the clouds, and the passing along of the carriages does raise that sound? Far be it. This is boyish, vain, trifling. What is then, “The voice of Your thunder is in the wheel”? Your voice rolls. Not even this do I understand. What shall we do? Let us question Idithun himself, to see whether perchance he may himself explain what he has said: “The voice,” he says, “of Your thunder is in the wheel.” I do not understand. I will hear what you say, “Your lightnings have appeared to the round world.” Say then, I had no understanding. The round world is a wheel. For the circuit of the round world is with reason called also an “orb:” whence also a small wheel is called an “orbiculus.” “The voice of Your thunder is in the wheel:” Your “lightnings have appeared to the round world.” Those clouds in a wheel have gone about the round world, have gone about with thundering and with lightning, they have shaken the abyss, with commandments they have thundered, with miracles they have lightened. “Unto every land has gone forth the sound of them, and unto the ends of the orb the words of them.” “The land has been moved and made to tremble:” that is, all men that dwell in the land. But by a figure the land itself is sea. Why? Because all nations are called by the name of sea, inasmuch as human life is bitter, and exposed to storms and tempests. Moreover if you observe this, how men devour one another like fishes, how the stronger does swallow up the weaker— it is then a sea, unto it the Evangelists went.
19. “Your way is in the sea”. But now Your way was in the Holy One, now “Your way is in the sea:” because the Holy One Himself is in the sea, and with reason even did walk upon the waters of the sea. “Your way is in the sea,” that is, Your Christ is preached among the Gentiles....“Your way is in the sea, and Your paths in many waters,” that is, in many peoples. “And Your footsteps will not be known.” He has touched certain, and wonder were it if it be not those same Jews. Behold now the mercy of Christ has been so published to the Gentiles, that “Your way is in the sea. Your footsteps will not be known.” How so, by whom will they not be known, save by those who still say, Christ has not yet come? Why do they say, Christ has not yet come? Because they do not yet recognise Him walking on the sea.
20. “You have led home Your people like sheep in the hand of Moses and of Aaron”. Why He has added this is somewhat difficult to discover....They banished Christ; sick as they were, they would not have Him for their Saviour; but He began to be among the Gentiles, and among all nations, among many peoples. Nevertheless, a remnant of that people has been saved. The ungrateful multitude has remained without, even the halting breadth of Jacob's thigh. For the breadth of the thigh is understood of the multitude of lineage, and among the greater part of the Israelites a certain multitude became vain and foolish, so as not to know the steps of Christ on the waters. “You have led home Your people like sheep,” and they have not known You. Though You have done such great benefits unto them, hast divided sea, hast made them pass over dry land between waters, hast drowned in the waves pursuing enemies, in the desert hast rained manna for their hunger, leading them home “by the hand of Moses and Aaron:” still they thrust You from them, so that in the sea was Your Way, and Your steps they knew not.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)