9 Wherefore what has He done by the terror of His counsel? He has turned the sea into dry land. For this follows, “That has turned the sea into dry land”. A sea was the world, bitter with saltness, troubled with tempest, raging with waves of persecutions, sea it was: truly into dry land the sea has been turned, now there thirsts for sweet water the world that with salt water was filled. Who has done this? He “that has turned the sea into dry land.” Now the soul of all the Gentiles says what? “My soul is as it were land without water to You.” “That has turned the sea into dry land. In the river they shall pass over on foot.” Those same persons that have been turned into dry land, though they were before sea, “in the river on foot shall pass over.” What is the river? The river is all the mortality of the world. Observe a river: some things come and pass by, other things that are to pass by do succeed. Is it not thus with the water of a river, that from earth springs and flows? Every one that is born must needs give place to one going to be born: and all this order of things rolling along is a kind of river. Into this river let not the soul greedily throw herself, let her not throw herself, but let her stand still. And how shall she pass over the pleasures of things doomed to perish? Let her believe in Christ, and she will pass over on foot: she passes over with Him for Leader, on foot she passes over.
10. “There we will be joyous in Him.” O you Jews, of your own works boast ye: lay aside the pride of boasting of yourselves, take up the Grace of being joyous in Christ. For therein we will be joyous, but not in ourselves: “there we will be joyous in Him.” When shall we joy? When we shall have passed over the river on foot. Life everlasting is promised, resurrection is promised, there our flesh no longer shall be a river: for a river it is now, while it is mortality. Observe whether there stands still any age. Boys desire to grow up; and they know not how by succeeding years the span of their life is lessened. For years are not added to but taken from them as they grow: just as the water of a river always draws near, but from the source it withdraws. And boys desire to grow up that they may escape the thraldom of elders; behold they grow up, it comes to pass quickly, they arrive at youth: let them that have emerged from boyhood retain, if they are able, their youth: that too passes away. Old age succeeds: let even old age be everlasting; with death it is removed. Therefore a river there is of flesh that is born. This river of mortality, so that it does not by reason of concupiscence of things mortal undermine and carry him away, he easily passes over, that humbly, that is on foot, passes over, He being leader that first has passed over, that of the flood in the way even unto death has drunk, and therefore has lifted up the head. Passing over therefore on foot that river, that is, easily passing over that mortality that glides along, “there we will be joyous in Him.” But now in what save in Him, or in the hope of Him? For even if we are joyous now, in hope we are joyous; but then in Him we shall be joyous. And now in Him, but through hope: “but then face to face.” “There we will be joyous in Him.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)