Though the waters thereof roar, etc This verse ought to be read in connection with the verse which follows, because it is necessary to complete the sense, as if it had been said: Though the waters of the sea roar and swell, and by their fierce impetuosity shake the very mountains -- even in the midst of these dreadful tumults, the holy city of God will continue to enjoy comfort and peace, satisfied with her small streams. The relative pronoun her, according to the common usage of the Hebrew language, is superfluous in this place. The prophet intended simply to say, that the small streams of a river would afford to the holy city abundant cause of rejoicing, though the whole world should be moved and destroyed. I have already mentioned shortly before how profitable is the doctrine taught us in this place, that our faith is really and truly tested only when we are brought into very severe conflicts, and when even hell itself seems opened to swallow us up. In like manner, we have portrayed to us the victory of faith over the whole world, when, in the midst of the utmost confusion, it unfolds itself, and begins to raise its head in such a manner as that although the whole creation seem to be banded together, and to have conspired for the destruction of the faithful, it nevertheless triumphs over all fear. Not that the children of God, when placed in peril, indulge in jesting or make a sport of death, but the help which God has promised them more than overbalances, in their estimation, all the evils which inspire them with fear. The sentiment of Horace is very beautiful, when, speaking of the righteous man and the man who feels conscious of no guilt, he says, (Car., Lib. iii., Od. 3,) "Dux inquieti turbidus Adriae, Nec fulminantis magna Jovis manus, Si fractus illabitur orbis, Impavidum ferient ruinae." "Let the wild winds that rule the seas, Tempestuous, all their horrors raise; Let Jove's dread arm with thunders rend the spheres; Beneath the crush of worlds undaunted he appears." [1] But as no such person as he imagines could ever be found, he only trifles in speaking as he does. Their fortitude, therefore, has its foundation in the assurance of the divine protection alone, so that they who rely upon God, and put their trust in him, may truly boast, not only that they shall be undismayed, but also that they shall be preserved in security and safety amidst the ruins of a falling world. The prophet says expressly, that the city of God shall be glad, although it had no raging sea, but only a gently flowing stream, to set for its defense against those waves of which he has made mention. By this mode of expression he alludes to the stream which flowed from Shiloah, and passed through the city of Jerusalem. Further, the prophet, I have no doubt, here indirectly rebukes the vain confidence of those who, fortified by earthly assistance, imagine that they are well protected, and beyond the reach of all danger. Those who anxiously seek to strengthen themselves on all sides with the invincible helps of the world, seem, indeed, to imagine that they are able to prevent their enemies from approaching them, just as if they were environed on all sides with the sea; but it often happens that the very defenses which they had reared turn to their own destruction, even as when a tempest lays waste and destroys an island by overflowing it. But they who commit themselves to the protection of God, although in the estimation of the world they are exposed to every kind of injury, and are not sufficiently able to repel the assaults made upon them, nevertheless repose in security. On this account, Isaiah (Isaiah 8:6) reproves the Jews because they despised the gently flowing waters of Shiloah, and longed for deep and rapid rivers. In that passage, there is an elegant antithesis between the little brook Shiloah on the one hand, and the Nile and Euphrates on the other; as if he had said, They defraud God of his honor by the unworthy reflection, that when he made choice of the city of Jerusalem, he had not made the necessary provision in respect of strength and fortifications for its defense and preservation. And certainly, if this psalm was written after the slaughter and flight of the army of Sennacherib, it is probable that the inspired writer purposely made use of the same metaphor, to teach the faithful in all ages, that the grace of God alone would be to them a sufficient protection, independent of the assistance of the world. In like manner, the Holy Spirit still exhorts and encourages us to cherish the same confidence, that, despising all the resources of those who proudly magnify themselves against us, we may preserve our tranquillity in the midst of disquietude and trouble, and not be grieved or ashamed on account of our defenseless condition, so long as the hand of God is stretched out to save us. Thus, although the help of God comes to our aid in a secret and gentle manner, like the still flowing streams, yet it imparts to us more tranquillity of mind than if the whole power of the world were gathered together for our help. In speaking of Jerusalem as the sanctuary of the tabernacles of the Most High, the prophet makes a beautiful allusion to the circumstances or condition of that time: for although God exercised authority over all the tribes of the people, yet he made choice of that city as the seat of royalty, from which he might govern the whole nation of Israel. The tabernacles of the Most High were scattered throughout all Judea, but still it was necessary that they should be gathered together and united in one sanctuary, that they might be under the dominion of God.

Footnotes

1 - Francis' Translation of Horace.

Though the waters thereof roar and be troubled - The waters of the sea. The idea is, that they would not be afraid though everything should be in commotion, and be as unsettled as the restless waves of the ocean. The earth might be changed, the mountains removed, the agitated sea roar and dash against the shore, but their minds would be calm. The word rendered "be troubled" means to boil; to ferment; to foam; and here it refers to the ocean as agitated and lashed into foam. Nothing is more sublime and fearful than the ocean in a storm; nothing furnishes a better illustration of the peace produced by confidence in God amid the agitations which occur in the world, than the mind of a seaman that is calm when the ocean is heaved in wild commotion.
Though the mountains shake with the swelling thereof - The rolling ocean breaking against; the sides of the mountains on its shore, and seeming to shake them to their foundation. The word rendered "swelling" means properly majesty, glory; then pride, haughtiness, insolence. Literally, "though the mountains tremble through their pride." Compare Psalm 124:5. On the word "Selah," see the notes at Psalm 3:2.

Though the waters thereof roar - Waters, in prophetic language, signify people; and, generally, people in a state of political commotion, here signified by the term roar. And by these strong agitations of the people, the mountains - the secular rulers, shake with the swelling thereof - tremble, for fear that these popular tumults should terminate in the subversion of the state. This very people had seen all Asia in a state of war. The Persians had overturned Asia Minor, and destroyed the Babylonian empire: they had seen Babylon itself sacked and entered by the Persians; and Cyrus, its conqueror, had behaved to them as a father and deliverer. While their oppressors were destroyed, themselves were preserved, and permitted to return to their own land.

[Though] the waters thereof (d) roar [and] be troubled, [though] the mountains shake with the swelling thereof. Selah.
(d) Though the afflictions rage, yet the rivers of God's mercies bring sufficient comfort to his.

Though the waters thereof roar and be troubled,.... The noise of which causes men's hearts to fail them for fear, Luke 21:25;
though the mountains shake with the swelling thereof. All these figurative expressions denote the hurlyburlies, confusions, and disorders that have been or will be in the world; amidst all which the people of God have no reason to fear; for it is always well with the righteous, let it go how it will with others. The passage may be applied to the destruction of Jerusalem, and the wars preceding it, and the dispersion of the Jews upon it; when true believers in Christ found him to be their refuge, strength, and help in that time of trouble, such as never was the like, and were safe and without fear; and Aben Ezra, a Jewish commentator, thinks it is right to interpret this psalm concerning the wars of Jerusalem: moreover, these words may be applied to any other time of calamity, through war or persecution, that has been since; as also to any that is to come; as to the slaying of the witnesses, the hour of temptation that will try all that are upon the earth; and even to the day of judgment, when heaven and earth shall flee away from the face of the Judge; when the heavens shall be folded up as a garment, and the earth, and all that is therein, shall be burnt up, and the whole world of the ungodly shall be thrown into the utmost panic, the saints will be safe with Christ, and ever happy with him; and, in the worst of times in this world, God is always their covenant God, their shield, portion, and exceeding great reward; Christ is their Redeemer and Saviour, their city of refuge, and strong hold; and though they may be plundered of their goods and property, they have a better and a more enduring substance in heaven; an estate, an inheritance there, that can never be taken away; and even should their enemies kill the body, that is the utmost they can do; their souls are safe in the hands of Christ; their life is hid with him; and when he shall appear, they shall appear with him in glory; and therefore they may well say, "we will not fear" (w).
Selah; on this word; see Gill on Psalm 3:2.
(w) "Si fractus illabatur orbis", &c. Horat. Carmin. l. 3. Ode. 3. v. 7.

swelling--well represents the pride and haughtiness of insolent foes.