3 “I will sing to the harp, and will have understanding, in the spotless way. When You shall come unto me”. Except in the spotless way, you can neither sing to the harp, nor understand. If you dost wish to understand, sing in the spotless way, that is, work with cheerfulness before your God. What is the spotless way? Hear what follows: “I walked in innocence, in the midst of my house.” This spotless way begins from innocence, and it ends also in innocence. Why seek many words?
Be innocent: and you have perfected righteousness....But who is innocent? He who while he hurts not another, injures not himself. For he who hurts himself, is not innocent. Some one says: Lo, I have not robbed any one, I have not oppressed any one: I will live happily on my own substance, the fruits of my virtuous toil; I wish to have fine banquets, I wish to spend as much as pleases me, to drink with those whom I like as much as I please; whom have I robbed, whom have I oppressed, who has complained of me?
He seems innocent. But if he corrupt himself, if he overthrow the temple of God within himself, why hope that he will act with mercy toward others, and spare the wretched? Can that man be merciful to others, who unto himself is cruel? The whole of righteousness, therefore, is reduced to the one word, innocence. But the lover of iniquity, hates his own soul. When he loved iniquity, he fancied he was injuring others. But consider whether he was injuring others: “He who loves iniquity,” he says, “hates his own soul.” He therefore who wishes to injure another, first injures himself; nor does he walk, since there is no room.
For all wickedness suffers from narrowness: innocence alone is broad enough to walk in. “I walked in the innocence of my heart, in the midst of my house.” By the middle of his house, he either signifies the Church herself; for Christ walks in her: or his own heart; for our inner house is our heart: as he has explained in the above words, “in the innocence of my heart.” What is the innocence of the heart? The middle of his house? Whoever has a bad house in this, is driven out of doors.
For whoever is oppressed within his heart by a bad conscience, just as any man in consequence of the overflow of a waterspout or of smoke goes out of his house, suffers not himself to dwell therein; so he who has not a quiet heart, cannot happily dwell in his heart. Such men go out of themselves in the bent of their mind, and delight themselves with things without, that affect the body; they seek repose in trifles, in spectacles, in luxuries, in all evils. Wherefore do they wish themselves well without? Because it is not well with them within, so that they may rejoice in a good conscience....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)