What is the New Law of the Gospel, of the New Testament? JPII writes in VS §24 that it is the grace of the Holy Spirit poured out in our hearts.
And so we find revealed the authentic and original aspect of the commandment of love and of the perfection to which it is ordered: we are speaking of a possibility opened up to man exclusively by grace, by the gift of God, by his love. On the other hand, precisely the awareness of having received the gift, of possessing in Jesus Christ the love of God, generates and sustains the free response of a full love for God and the brethren, as the Apostle John insistently reminds us in his first Letter: “Beloved, let us love one another; for love is of God and knows God. He who does not love does not know God; for God is love… Beloved, if God so loved us, we ought also to love one another… We love, because he first loved us” (1 Jn 4:7-8, 11, 19).
This inseparable connection between the Lord’s grace and human freedom, between gift and task, has been expressed in simple yet profound words by Saint Augustine in his prayer: “Da quod iubes et iube quod vis” (grant what you command and command what you will).
The gift does not lessen but reinforces the moral demands of love: “This is his commandment, that we should believe in the name of his Son Jesus Christ and love one another just as he has commanded us” (1 Jn 3:32). One can “abide” in love only by keeping the commandments, as Jesus states: “If you keep my commandments, you will abide in my love, just as I have kept my Father’s commandments and abide in his love” (Jn 15:10).
Going to the heart of the moral message of Jesus and the preaching of the Apostles, and summing up in a remarkable way the great tradition of the Fathers of the East and West, and of Saint Augustine in particular, Saint Thomas was able to write that the New Law is the grace of the Holy Spirit given through faith in Christ. The external precepts also mentioned in the Gospel dispose one for this grace or produce its effects in one’s life. Indeed, the New Law is not content to say what must be done, but also gives the power to “do what is true” (cf. Jn 3:21). [Emphasis in original]
He refers to the following passage from the Summa Theologiae:
“Each thing appears to be that which preponderates in it,” as the Philosopher states (Ethic. ix, 8). Now that which is preponderant in the law of the New Testament, and whereon all its efficacy is based, is the grace of the Holy Ghost, which is given through faith in Christ. Consequently the New Law is chiefly the grace itself of the Holy Ghost, which is given to those who believe in Christ. This is manifestly stated by the Apostle who says (Romans 3:27): “Where is . . . thy boasting? It is excluded. By what law? Of works? No, but by the law of faith”: for he calls the grace itself of faith “a law.” And still more clearly it is written (Romans 8:2): “The law of the spirit of life, in Christ Jesus, hath delivered me from the law of sin and of death.” Hence Augustine says (De Spir. et Lit. xxiv) that “as the law of deeds was written on tables of stone, so is the law of faith inscribed on the hearts of the faithful”: and elsewhere, in the same book (xxi): “What else are the Divine laws written by God Himself on our hearts, but the very presence of His Holy Spirit?”
Nevertheless the New Law contains certain things that dispose us to receive the grace of the Holy Ghost, and pertaining to the use of that grace: such things are of secondary importance, so to speak, in the New Law; and the faithful need to be instructed concerning them, both by word and writing, both as to what they should believe and as to what they should do. Consequently we must say that the New Law is in the first place a law that is inscribed on our hearts, but that secondarily it is a written law. [ST I-II q106 a1]
This law is fundamental to Christian morality, in that by it we learn not only what we must do, but are both empowered and disposed to do it. Consequently we see that not only are we not able to save ourselves—for we are saved by grace—but we are also incapable of living the Christian life except by God’s grace.