Chapter 36 [XXI.]—The Law Written in Our Hearts.
What then is God’s law written by God Himself in the hearts of men, but the very presence of the Holy Spirit, who is “the finger of God,” and by whose presence is shed abroad in our hearts the love which is the fulfilling of the law, and the end of the commandment? Now the promises of the Old Testament are earthly; and yet (with the exception of the sacramental ordinances which were the shadow of things to come, such as circumcision, the Sabbath and other observances of days, and the ceremonies of certain meats, and the complicated ritual of sacrifices and sacred things which suited “the oldness” of the carnal law and its slavish yoke) it contains such precepts of righteousness as we are even now taught to observe, which were especially expressly drawn out on the two tables without figure or shadow: for instance, “Thou shalt not commit adultery,” “Thou shalt do no murder,” “Thou shalt not covet,” “and whatsoever other commandment is briefly comprehended in the saying, Thou shall love thy neighbour as thyself.” Nevertheless, whereas as in the said Testament earthly and temporal promises are, as I have said, recited, and these are goods of this corruptible flesh (although they prefigure those heavenly and everlasting blessings which belong to the New Testament), what is now promised is a good for the heart itself, a good for the mind, a good of the spirit, that is, an intellectual good; since it is said, “I will put my law in their inward parts, and in their hearts will I write them,” —by which He signified that men would not fear the law which alarmed them externally, but would love the very righteousness of the law which dwelt inwardly in their hearts.
Reproduced in full. Translation: Nicene and Post-Nicene Fathers, series 1, vol. 5 (Schaff, 1887), public domain (Public domain).