17 If somewhat too prolix we have been, pardon us. We exhort you in the name of Christ, to meditate profitably on those things which you have heard. Because even to preach the truth is nought, if heart from tongue dissents; and to hear the truth nothing profits, if a man upon the rock build not. He that builds upon a Rock, is the same that hears and does: but he that hears and does not, builds upon sand: he that neither hears nor does, builds nothing....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)