6 Faustus does not understand, or pretends not to understand, what it is to fulfill the law. He supposes the expression to mean the addition of words to the law, regarding which it is written that nothing is to be added to or taken away from the Scriptures of God. From this Faustus argues that there can be no fulfillment of what is spoken of as so perfect that nothing can be added to it or taken from it. Faustus requires to be told that the law is fulfilled by living as it enjoins.
"Love is the fulfilling of the law," as the apostle says. The Lord has vouchsafed both to manifest and to impart this love, by sending the Holy Spirit to His believing people. So it is said by the same apostle: "The love of God is shed abroad in our heart by the Holy Ghost, which is given unto us." And the Lord Himself says: "By this shall all men know that you are my disciples, if you have love one to another." The law, then, is fulfilled both by the observance of its precepts and by the accomplishment of its prophecies.
For "the law was given by Moses, but grace and truth came by Jesus Christ." The law itself, by being fulfilled, becomes grace and truth. Grace is the fulfillment of love, and truth is the accomplishment of the prophecies. And as both grace and truth are by Christ, it follows that He came not to destroy the law, but to fulfill it; not by supplying any defects in the law, but by obedience to what is written in the law. Christ's own words declare this. For He does not say, One jot or one tittle shall in no wise pass from the law till its defects are supplied, but "till all be fulfilled."
Source: Reply to Faustus the Manichaean (New Advent)