11 “That troublest the bottom of the sea”. He has done this: it is seen what He has done. For He has prepared mountains in His strength, has sent them to preach: girded about He is by believers in power: and moved is the sea, moved is the world, and it begins to persecute His saints. “Girded about in power: that troublest the bottom of the sea.” He has not said, that troublest the sea; but “the bottom of the sea.” The bottom of the sea is the heart of ungodly men. For just as from the bottom more thoroughly all things are stirred, and the bottom holds firm all things: so whatsoever has gone forth: by tongue, by hands, by various powers for the persecution of the Church, from the bottom has gone forth.
For if there were not the root of iniquity in the heart, all those things would not have gone forth against Christ. The bottom He troubled, perchance in order that the bottom He might also empty: for in the case of certain evil men He emptied the sea from the bottom, and made the sea a desert place. Another Psalm says this, “That turns sea into dry land.” All ungodly and heathen men that have believed were sea, have been made land; with salt waves at first barren, afterwards with the fruit of righteousness productive.
“That troublest the bottom of the sea: the sound of its waves who shall endure?” “Who shall endure,” is what? What man shall endure the sound of the waves of the sea, the behests of the high powers of the world? But whence are they endured? Because He prepares mountains in His strength. In that therefore which he has said “who shall” endure he says thus: We ourselves of our own selves should not be able to endure those persecutions, unless He gave strength.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)