To love our neighbor's welfare as much as our own: that is true and sincere charity out of a pure heart, and of a good conscience, and of faith unfeigned (I Tim. 1:5). Whosoever loves his own prosperity only is proved thereby not to love good for its own sake, since he loves it on his own account. And so he cannot sing with the psalmist, 'O give thanks unto the Lord, for He is gracious' (Ps. 118:1). Such a man would praise God, not because He is goodness, but because He has been good to him: he could take to himself the reproach of the same writer, 'So long as Thou doest well unto him, he will speak good of Thee' (Ps. 49:18, Vulg.). One praises God because He is mighty, another because He is gracious, yet another solely because He is essential goodness. The first is a slave and fears for himself; the second is greedy, desiring further benefits; but the third is a son who honors his Father. He who fears, he who profits, are both concerned about self-interest. Only in the son is that charity which seeketh not her own (I Cor. 13:5). Wherefore I take this saying, 'The law of the Lord is an undefiled law, converting the soul' (Ps. 19:7) to be of charity; because charity alone is able to turn the soul away from love of self and of the world to pure love of God. Neither fear nor self-interest can convert the soul. They may change the appearance, perhaps even the conduct, but never the object of supreme desire. Sometimes a slave may do God's work; but because he does not toil voluntarily, he remains in bondage. So a mercenary may serve God, but because he puts a price on his service, he is enchained by his own greediness. For where there is self-interest there is isolation; and such isolation is like the dark corner of a room where dust and rust befoul. Fear is the motive which constrains the slave; greed binds the selfish man, by which he is tempted when he is drawn away by his own lust and enticed (James 1:14). But neither fear nor self-interest is undefiled, nor can they convert the soul. Only charity can convert the soul, freeing it from unworthy motives.
Next, I call it undefined because it never keeps back anything of its own for itself. When a man boasts of nothing as his very own, surely all that he has is God's; and what is God's cannot be unclean. The undefiled law of the Lord is that love which bids men seek not their own, but every man another's wealth. It is called the law of the Lord as much because He lives in accordance with it as because no man has it except by gift from Him. Nor is it improper to say that even God lives by law, when that law is the law of love. For what preserves the glorious and ineffable Unity of the blessed Trinity, except love? Charity, the law of the Lord, joins the Three Persons into the unity of the Godhead and unites the holy Trinity in the bond of peace. Do not suppose me to imply that charity exists as an accidental quality of Deity; for whatever could be conceived of as wanting in the divine Nature is not God. No, it is the very substance of the Godhead; and my assertion is neither novel nor extraordinary, since St. John says, 'God is love' (I John 4:8). One may therefore say with truth that love is at once God and the gift of God, essential love imparting the quality of love. Where the word refers to the Giver, it is the name of His very being; where the gift is meant, it is the name of a quality. Love is the eternal law whereby the universe was created and is ruled. Since all things are ordered in measure and number and weight, and nothing is left outside the realm of law, that universal law cannot itself be without a law, which is itself. So love though it did not create itself, does surely govern itself by its own decree.