Chapter 29 [XVII.]— A Comparison of the Law of Moses and of the New Law
Now, amidst this admirable correspondence, there is at least this very considerable diversity in the cases, in that the people in the earlier instance were deterred by a horrible dread from approaching the place where the law was given; whereas in the other case the Holy Ghost came upon them who were gathered together in expectation of His promised gift. There it was on tables of stone that the finger of God operated; here it was on the hearts of men. There the law was given outwardly, so that the unrighteous might be terrified; here it was given inwardly, so that they might be justified. For this, “You shall not commit adultery, You shall not kill, You shall not covet; and if there be any other commandment,”— such, of course, as was written on those tables—“it is briefly comprehended,” says he, “in this saying, namely, You shall love your neighbour as yourself.
Love works no ill to his neighbour: therefore love is the fulfilling of the law.” Now this was not written on the tables of stone, but “is shed abroad in our hearts by the Holy Ghost, which is given unto us.” God's law, therefore, is love. “To it the carnal mind is not subject, neither indeed can be;” but when the works of love are written on tables to alarm the carnal mind, there arises the law of works and “the letter which kills” the transgressor; but when love itself is shed abroad in the hearts of believers, then we have the law of faith, and the spirit which gives life to him that loves.
Source: On the Spirit and the Letter (New Advent)