Psalms 37:31

(Vers. 31.) Justice is still described and increased in form, as the Scripture says: The law of God is in his heart, and his steps will not be supplanted. In the heart of the righteous is the law of God. What law? Not written, but natural; for the law is not imposed on the just, but on the unjust. In his heart is the law, not superficially, as in the lips of the Jews; for it is believed in the heart for righteousness. He who believes, speaks; but he who speaks does not always believe.

(Verse 30 and following) When he established the just moral disciplines, to teach you what kind you ought to be, and what would be the perfect form of justice; he wants his mind to rise to wisdom and theorems; and arouse his intention, so that he may look at heavenly things with an attentive heart, and revolve the divine oracles within himself, and direct his affection towards those things which please God: let him meditate on the Law, and let no commandments of the Lord escape him; let him recognize the movement of the divine Sacrament. Finally, I saw in the teachings that the holy Prophet arose: The mouth of the just man will meditate wisdom, and his tongue will speak judgment. The law of God is in his heart; the Lord will not condemn him when he is judged. Wait for the Lord and keep his way; and he will exalt you to possess the land: when sinners are destroyed, you will see. I saw the wicked man exalted and lifted up above the cedars of Lebanon; and I passed by, and behold, he was no more; and I sought him, but his place was not found. Superior moral things, these are intelligible. For what is the just mouth that will contemplate wisdom, if not the inner man? For there are two men in each person: one inner, the other outer; the inner one thinks about things of the mind, speaks about things of the mind; the outer one about things of the body. However, the coming Lord united both; and he established two in one person; so that they would not oppose each other with conflicting movements, but rather be united to each other by the unity of their wills. Therefore, the mouth of the righteous will meditate wisdom. For now even the outer man of the righteous is transformed into the discipline of the inner man, and is conformed to its nature, and performs its duties, so that the flesh may meditate on what is of the inner mind. […]

"The law of his God is in his heart" [Psalm 37:31]. Lest haply you should think him to have that on his lips, which he has not in his heart, lest you should reckon him among those of whom it is said, "This people honour Me with their lips, but their heart is far from Me." [Isaiah 29:13] And of what use is this to him? "And none of his steps shall slide." The "word of God in the heart" frees from the snare; the "word of God in the heart" delivers from the evil way; "the word of God in the heart" delivers from "the slippery place." He is with you, Whose word departs not from you. Now what evil does he suffer, whom God keeps? Thou settest a watchman in your vineyard, and feelest secure from thieves; and that watchman may sleep, and may himself fall, and may admit a thief. But "He who keeps Israel shall neither slumber nor sleep." "The law of his God is in his heart, and none of his steps shall slide." Let him therefore live free from fear; let him live free from fear even in the midst of the wicked; free from fear even in the midst of the ungodly. For what evil can the ungodly or unrighteous man do to the righteous? Lo! see what follows.

It is fitting, the psalmist says, to carry around the divine sayings both on one's tongue and in one's mind and constantly give attention to them; learning in this way how to behave, the lover of virtue remains intrepid and unmoved, proof against efforts at …

"The mouth of the just shall meditate." Here he shows that the goods of the just are greater with regard to the exercise of virtue, because the unjust have a mouth prone to evil. First, therefore, he treats of the spiritual goods of the just. …