13 Brethren, let not your instruments of music rest in your work: sing one to another songs of Sion. Readily have ye heard; the more readily do what you have heard, if you wish not to be willows of Babylon fed by its streams, and bringing no fruit. But sigh for the everlasting Jerusalem: whither your hope goes before, let your life follow; there we shall be with Christ. Christ now is our Head; now He rules us from above; in that city He will fold us to Himself; we shall be equal to the Angels of God.
We should not dare to imagine this of ourselves, did not the Truth promise it. This then desire, brethren, this day and night think on. Howsoever the world shine happily on you, presume not, parley not willingly with your lusts. Is it a grown-up enemy? Let it be slain upon the Rock. Is it a little enemy? Let it be dashed against the Rock. Slay the grown-up ones on the Rock, and dash the little ones against the Rock. Let the Rock conquer. Be built upon the Rock, if you desire not to be swept away either by the stream, or the winds, or the rain.
If you wish to be armed against temptations in this world, let longing for the everlasting Jerusalem grow and be strengthened in your hearts. Your captivity will pass away, your happiness will come; the last enemy shall be destroyed, and we shall triumph with our King, without death.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)