10 What is, “In the midst between the mountains the waters shall pass through”? We have heard who are the “mountains,” the great Preachers of the word, the exalted Angels of God, though still in mortal flesh; lofty not by their own power, but by His grace; but as far as relates to themselves, they are valleys, in their humility they send forth springs. “In the midst,” he says, “between the mountains, the waters shall pass through.” Let us suppose this said thus, “In the midst between the Apostles shall pass through the preachings of the Word of Truth.”
What is, in the midst between the Apostles? What is called in the midst, is common. A common property, from which all alike live, is in the midst, and belongs not to me, but neither belongs it to you, nor yet to me....For if they are not in the midst, they are as it were private, they flow not for public use, and I have mine, and he has his own, it is not in the midst for both me and him to have it; but such is not the preaching of peace....Therefore, brethren, let what we have said to your Love serve to this purpose, because of the springs: that they may flow from you, be ye valleys, and communicate with all that which you have from God.
Let the waters flow in the midst, envy ye no one, drink, be filled, flow forth when you are filled. Everywhere let the common water of God have the glory, not the private falsehoods of men....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)