Along the same line of the previous post, I have included the following quotations from St. Augustine.
Therefore, the apostle is not to be confined to the limited application just mentioned, when he says, "The letter killeth, but the spirit giveth life;" but this is also (and indeed especially) equivalent to what he says elsewhere in the plainest words: "I had not known lust, except the law had said, Thou shalt not covet;" and again, immediately after: "Sin, taking occasion by the commandment, deceived me, and by it slew me." Now from this you may see what is meant by "the letter that killeth." There is, of course, nothing, said figuratively which is not to be accepted in its plain sense, when it is said, "Thou shall not covet;" but this is a very plain and salutary precept, and any man who shall fulfil it will have I no sin at all. The apostle, indeed, purposely selected this general precept, in which he embraced everything, as if this were the voice of the law, prohibiting us from all sin, when he says, "Thou shalt not covet;" for there is no sin committed except by evil concupiscence; so that the law which prohibits this is a good and praiseworthy law. But, when the Holy Ghost withholds His help, which inspires us with a good desire instead of this evil desire (in other words, diffuses love in our hearts), that law, however good in itself, only
augments the evil desire by forbidding it. Just as the rush of water which flows incessantly in a particular direction, becomes more violent when it meets with any impediment, and when it has overcome the stoppage, falls in a greater bulk, and with increased impetuosity hurries forward in its downward course. In some strange way the very object which we covet becomes all the more pleasant when it is forbidden. [St. Augustine of Hippo, On the Spirit and the Letter, Chapter 6]
"Now the Lord is that Spirit: and where the Spirit of the Lord is, there is liberty." Now this Spirit of God, by whose gift we are justified, whence it comes to pass that we delight not to sin, -- in which is liberty; even as, when we are without this Spirit, we delight to sin, -- in which is slavery, from the works of which we must abstain; -- this Holy Spirit, through whom love is shed abroad in our hearts, which is the fulfilment of the law, is designated in the gospel as "the finger of God." Is it not because those very tables of the law were written by the finger of God, that the Spirit of God by whom we are sanctified is also the finger of God, in order that, living by faith, we may do good works through love? [ibid, Chapter 28]
Regarding the Law written upon 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? [ibid, Chapter 36]