The law of the Gospel, called the New Law

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?”

The Apostle says (Romans 1:16): “I am not ashamed of the Gospel: for it is in the power of God unto salvation to everyone that believeth.” But there is no salvation but to those who are justified. Therefore the Law of the Gospel justifies.

I answer that, As stated above (Article 1), there is a twofold element in the Law of the Gospel. There is the chief element, viz. the grace of the Holy Ghost bestowed inwardly. And as to this, the New Law justifies. Hence Augustine says (De Spir. et Lit. xvii): “There,” i.e. in the Old Testament, “the Law was set forth in an outward fashion, that the ungodly might be afraid”; “here,” i.e. in the New Testament, “it is given in an inward manner, that they may be justified.” The other element of the Evangelical Law is secondary: namely, the teachings of faith, and those commandments which direct human affections and human actions. And as to this, the New Law does not justify. Hence the Apostle says (2 Corinthians 3:6) “The letter killeth, but the spirit quickeneth”: and Augustine explains this (De Spir. et Lit. xiv, xvii) by saying that the letter denotes any writing external to man, even that of the moral precepts such as are contained in the Gospel. Wherefore the letter, even of the Gospel would kill, unless there were the inward presence of the healing grace of faith.

Thomas Aquinas, Summa Theologica ((I-II, Question 106, Articles 1&2)