From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
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).31
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,32 Saint Thomas was able to write that the New Law is the grace of the Holy Spirit given through faith in Christ.33 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). Saint John Chrysostom likewise observed that the New Law was promulgated at the descent of the Holy Spirit from heaven on the day of Pentecost, and that the Apostles “did not come down from the mountain carrying, like Moses, tablets of stone in their hands; but they came down carrying the Holy Spirit in their hearts . . . having become by his grace a living law, a living book”.34
31 Confessiones, X, 29, 40: Corpus Christianorum, Latin series 27, 176; cf. De Gratia et Livero Arbitrio, XV: Patrologiæ Cursus completus, Series Latina 44, 899.
32 Cf. De Spiritu et Littera, 21, 36; 26, 46: Corpus Scriptorum Ecclesiasticorum Latinorum 60, 189-190; 200-201.
33 Cf. Summa Theologiæ, I-II, q.106, a.1 conclusion and ad 2um.
34 In Matthæum, Hom. I,1: Patrologiæ Cursus completus, Series Græca 57,15.
— Veritatis Splendor 24