23 But see what follows, my brethren. For now some one or other, because God had said to him, “Immolate to God the sacrifice of praise,” and had enjoined in a manner this tribute, did meditate to himself and said, I will rise daily, I will proceed to Church, I will say one hymn at matins, another at vespers, a third or fourth in my house, daily I do sacrifice the sacrifice of praise, and immolate to my God. Well you do indeed, if you do this: but take heed, lest now thou be careless, because now you do this: and perchance your tongue bless God, and your life curse God.
O my people, says to you the God of gods, the Lord that spoke, “calling the earth from the rising of the sun unto the setting,” though yet you are placed amid the tares, “Immolate the sacrifice of praise to your God, and render to Him your prayers:” but take heed lest you live ill, and chant well. Wherefore this? For, “Unto the sinner, says God, why do you tell out My judgments, and takest My Covenant in your mouth?”. You see, brethren, with what trembling we say these words.
We take the Covenant of God in our mouth, and we say these words. We take the Covenant of God in our mouth, and we preach to you the instruction and judgment of God. And what says God to the sinner? “Why do you?” Does He then forbid preachers that be sinners? And where is that, “What they say do, but what they do, do not”? Where is that, “Whether in truth or on occasion Christ be preached”? But these words were said, lest they should fear that hear, from whomsoever it be that they hear: not that they should be without care that speak good words, and do evil deeds.
Now therefore, brethren, you are without care: if you hear good words ye hear God, through whomsoever it be that you may hear. But God would not dismiss without reproof them that speak: lest with their speaking alone, without care for themselves they should slumber in evil life, and say to themselves, “For God will not consign us to perdition, through whose mouth He has willed that so many good words should be spoken to His people.” Nay, but hear what you speak, whoever you are that speakest: and thou that writ be heard yourself, first hear yourself; and speak what a certain man does speak in another Psalm, “I will hear what in me speaks the Lord God, for He shall speak peace to His people.”
What am I then, that hear not what in me He speaks, and will that other hear what through me He speaks? I will hear first, will hear, and chiefly I will hear what speaks in me the Lord God, for He shall speak peace to His people. Let me hear, and “chasten my body, and to servitude subject it, lest perchance to others preaching, myself be found a cast-away.” “Why do you tell out my judgments?” Wherefore to you what profits not you? He admonishes him to hear: not to lay down preaching, but to take up obedience. “But you, why do you take My Covenant in they mouth?”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)