5 See what tranquillity: “Therefore will not we fear when the earth shall be confounded, and the mountains shall be carried into the heart of the sea.” Then we shall find not fear. Let us seek mountains carried, and if we can find, it is manifest that this is our security. The Lord truly said to His disciples, “If you have faith as a grain of mustard seed, you shall say to this mountain, Be Thou removed, and be Thou cast into the sea, and it shall be done.” Haply “to this mountain,” He said of Himself; for He is called a Mountain: “It shall come to pass in the last days, that the mountain of the Lord shall be manifest.” But this Mountain is placed above other mountains; because the Apostles also are mountains, supporting this Mountain. Therefore follows, “In the last days the Mountain of the Lord shall be manifest, established in the top of the mountains.” Therefore passes It the tops of all mountains, and on the top of all mountains is It placed; because the mountains are preaching The Mountain. But the sea signifies this world, in comparison of which sea, like earth seemed the nation of the Jews. For it was not covered over with the bitterness of idolatry, but, like dry land, was surrounded with the bitterness of the Gentiles as with sea. It was to be, that the earth be confounded, that is, that nation of the Jews; and that the mountains be carried into the heart of the sea, that is, first that great Mountain established in the top of the mountains. For He deserted the nation of the Jews, and came among the Gentiles. He was carried from the earth into the sea. Who carrying Him? The Apostles, to whom He had said, “If you have faith as a grain of mustard seed, you shall say to this mountain, Be thou removed, and be you cast into the sea, and it shall be done:” that is, through your most faithful preaching it shall come to pass, that this mountain, that is, I Myself, be preached among the Gentiles, be glorified among the Gentiles, be acknowledged among the Gentiles, and that be fulfilled which was predicted of Me, “A people whom I have not known shall serve Me.”...
6. “The waters thereof roared, and were troubled”: when the Gospel was preached, “What is this? He seems to be a setter forth of strange gods:” this the Athenians; but the Ephesians, with what tumult would they have slain the Apostles, when in the theatre, for their goddess Diana, they made such an uproar, as to be shouting, “Great is Diana of the Ephesians!” Amidst which waves and roaring of the sea, feared not they who to that refuge had fled. Nay, the Apostle Paul would enter in to the theatre, and was kept back by the disciples, because it was necessary that he should still abide in the flesh for their sakes. But yet, “the waters thereof roared, and were troubled: the mountains shook at the mightiness thereof.” Whose might? The sea's? Or rather God's, of whom was said, “refuge and strength, a helper in tribulations, which have found us out too much?” For shaken were the mountains, that is, the powers of this world. For one thing are the mountains of God, another the mountains of the world: the mountains of the world, they whose head is the devil, the mountains of God, they whose Head is Christ. But by these mountains were shaken those mountains. Then gave they their voices against Christians, when the mountains were shaken, the waters roaring; for the mountains were shaken, and there was made a great earthquake, with quaking of the sea. But against whom this? Against the City founded upon a rock. The waters roar, the mountains shake, the Gospel being preached. What then, the City of God? Hear what follows.
7. “The streams of the river make glad the City of God”. When the mountains shake, when the sea rages, God deserts not His City, by the streams of the river. What are these streams of the river? That overflowing of the Holy Spirit, of which the Lord said, “If any man thirst, let him come unto Me, and drink. He that believes in Me, out of his bosom shall flow rivers of living water.” These rivers then flowed out of the bosom of Paul, Peter, John, the other Apostles, the other faithful Evangelists. Since these rivers flowed from one river, many “streams of the river make glad the City of God.” For that you might know this to be said of the Holy Spirit, in the same Gospel next said the Evangelist, “But this spoke He of the Spirit, which they that were to believe in Him should receive. For the Holy Ghost was not yet given, because that Jesus was not yet glorified.” Jesus being glorified after His Resurrection, glorified after His Ascension, on the day of Pentecost came the Holy Spirit, and filled the believers, who spoke with tongues, and began to preach the Gospel to the Gentiles. Hence was the City of God made glad, while the sea was troubled by the roaring of its waters, while the mountains were confounded, asking what they should do, how drive out the new doctrine, how root out the race of Christians from the earth. Against whom? Against the streams of the river making glad the City of God. For thereby showed He of what river He spoke; that He signified the Holy Spirit, by “the streams of the river make glad the City of God.” And what follows? “The Most High has sanctified His tabernacle:” since then there follows the mention of Sanctification, it is manifest that these streams of the river are to be understood of the Holy Spirit, by whom is sanctified every godly soul believing in Christ, that it may be made a citizen of the City of God.
8. “God is in the midst of her: she shall not be moved”. Let the sea rage, the mountains shake; “God is in the midst of her: she shall not be moved.” What is, “in the midst of her”? That God stands in any one place, and they surround Him who believe in Him? Then is God circumscribed by place; and broad that which surrounds, narrow that which is surrounded? God forbid. No such thing imagine of God, who is contained in no place, whose seat is the conscience of the godly: and so is God's seat in the hearts of men, that if man fall from God, God in Himself abides, not falls like one not finding where to be. For rather does He lift up you, that you may be in Him, than so lean upon you, as if you withdraw yourself, to fall. Himself if He withdraw, fall will you: yourself if you withdraw, fall will not He. What then is, “God is in the midst of her”? It signifies that God is equal to all, and accepts not persons. For as that which is in the middle has equal distances to all the boundaries, so God is said to be in the middle, because He consults equally for all. “God is in the midst of her: she shall not be moved.” Wherefore shall she not be moved? Because God is in the midst of her. He is “the Helper in tribulations that have found us out too much. God shall help her with His Countenance.” What is, “with His Countenance”? With manifestation of Himself. How manifests God Himself, so as that we see His Countenance? I have already told you; you have learned God's Presence; we have learned it through His works. When from Him we receive any help so that we cannot at all doubt that it was granted to us by the Lord, then God's Countenance is with us.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)