2 “Halleluia.” “Praise the Lord,” you say to your neighbour, he to you: when all are exhorting each other, all are doing what they exhort others to do. But praise with your whole selves: that is, let not your tongue and voice alone praise God, but your conscience also, your life, your deeds. For now, when we are gathered together in the Church, we praise: when we go forth each to his own business, we seem to cease to praise God. Let a man not cease to live well, and then he ever praises God....It is impossible for a man's acts to be evil, whose thoughts are good.
For acts issue from thought: nor can a man do anything or move his limbs to do anything, unless the bidding of his thought precede: just as in all things which you see done throughout the provinces, whatsoever the Emperor bids goes forth from the inner part of his palace throughout the whole Roman Empire. How great commotion is caused at one bidding by the Emperor as he sits in his palace! He but moves his lips, when he speaks: the whole province is moved, when what he speaks is being executed.
So in each single man too, the Emperor is within, his seat is in the heart. If he be good and bids good things, good things are done: if he be bad and bids evil things, evil things are done. When Christ sits there, what can He bid, but what is good? When the devil is the occupant, what can he bid, but evil? But God has willed that it should be in your choice for whom you will prepare room, for God, or for the devil: when you have prepared it, he who is occupant will also rule. Therefore, brethren, attend not only to the sound; when you praise God, praise with your whole selves: let your voice, your life, your deeds, all sing.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)