"But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more." (Jer 31:33-34)
The temptation always is to think of law only as a set of rules to guide our behavior. But, in the more abstract, law is simply a proper ordering of things. For example, we talk about the laws of physics but do not mean by that a set of rules that physical matter may or may not obey. Rather, we mean the regular behavior of matter in the universe we live in. Inanimate matter has no choice in its behavior, of course, but is completely constrained by its very nature. Likewise, the laws of a just society represent a proper relationship between all the members of that society. Law should be a description of the natural behavior of man.
With man, having free will, it is possible to violate that proper relationship. And, because of this consciousness of choice, we begin to think of law as something external that we learn in order to judge our behavior and constrain our actions. Law is seen then as an external set of rules to constrain behavior. When we read in Jeremiah 31 that God will put His law in their inward parts, the common understanding would be that there is no need for the external law since the law is now internalized. Yet, that type of thinking fails to see the deepness of God's word here. Man still thinks of law as a constraining set of rules and misinterprets the whole Bible. The Bible is interpreted as though it were a system of rules of behavior to which we conform ourselves in order to gain a reward and avoid punishment. No wonder, then, that people reject the Bible in favor of some other set of laws that are more convenient to themselves.
But, I have always asked, "Why is it that we need to constrain our behavior?" The obvious answer would be that we need to constrain our actions so that we will do good and not do evil. Yet, that answer misses something important. It doesn't really answer the question of "why" but simply assumes restraint as necessary. So, I ask again, "why?" The answer really is that we have the wrong desires. This is the point Jesus makes in Matthew chapter five.
"Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire." (Mat 5:21-22)
The Pharisees, as most people, considered that they were fulfilling the law so long as their outward behavior was in conformance to the dictates of the law. Jesus expands and deepens the understanding of law by showing that it is not enough to conform your behavior to the law's dictates. If the corrupt desires, lust, greed, envy, hatred, etc., are in the heart, you are equally guilty. In this context you can understand Jesus' statement that he did not come to destroy the law and the prophets. His sayings here could be interpreted perversely as rejecting what the law said, but he is not contradicting the law. Rather, he is deepening, and filling-up, the meaning inherent in the law. The law was never about a system of conformity of behavior. It was always about exposing and constraining the corruption of man's nature.
If we could only desire the right things, and never desire the wrong things, law as we normally think of it would be irrelevant. You don't need restraints on moral desires; you only need to restrain that which is immoral. Get rid of the evil desire and the problem goes away completely. That really is the core problem of man. Having free will we can choose what is evil over what is good. Man, separated from God, will inevitably choose the wrong thing. No system, whether religious, political, or philosophical, changes the nature of man. All such systems can ever do is create a system of conformity to prescribed rules of behavior, reward those who conform, and punish those who don't. And thus, because nothing in the nature of man has changed, all will fail in due time.
The failure of religion is simply the same failure of human nature transferred into spiritual desires. Man thinks that the performance of the religious ritual, whatever form it may take, is the way he is deemed righteous and good. The religious ritual, however, never is an end of itself. This is the false idea that the Apostle Paul argues vehemently against in his epistles. No man is made righteous by works of the law. Meaning, the mere performance of ritual acts does not change the nature of man. Keeping the festivals, making sacrifices, eating the proper food, are not things that make man righteous. These things are types and shadows of what was to come and designed to teach man what God would do in the fullness of time. The law and its festivals are a promise of God to man with the ultimate fulfillment being the descent of God's presence (shekinah) into the heart of man as a living tabernacle. In effect, writing the law into the heart of man.
In his epistles, Paul represents the corrupt nature of man as the "flesh." It's a very appropriate term for it. The essence of the corrupt nature is that it always seeks its own survival, its own pleasure, and its own glory. From those self-serving desires come greed, envy, jealousy and hatred, and from those come all the immoral actions of man. It is this "flesh" that must die in order to rid ourselves of corruption. What must come to live in us is something other than carnal desire.
If the carnal desire is self-serving, then the opposite would be that desire that is other-serving. This laying down of ones own desires and seeking other-ness is what the Bible deems love. It is what God did for us. He poured out His life through Jesus that we might live. To love Him, then, is to pour out our life for His benefit. To have that same love in us is to then show that same love to one another.
"Owe no man any thing, but to love one another: for he that loveth another hath fulfilled the law. For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself. Love worketh no ill to his neighbour: therefore love is the fulfilling of the law." (Rom 13:8-10)
This is the meaning in Jeremiah 31. To have God's law in our inward parts, to fully know Him, is to have that same love in us that He shows towards us. Out of that love, which is the presence of God in us, all our desires and consequent actions will flow. That is the proper relationship both to God and to one another, and to be in proper relationship is the very definition of righteousness.
The one question remaining, the great mystery of Christianity, is how this new state of being comes about. Just as we cannot by our will power do what is right, we cannot transform our nature into love simply by desiring to do so. Some will attempt to say that it is simply a matter of ignorance. If we knew the right things we would do them. Yet, we lack the power to change ourselves even if we know with certainty that we ought to change. The change must come from outside us. As Jeremiah says, God will put His law of love into us. In the Gospels, it is described as partaking of the blood and body of Jesus Christ. Not in a literal sense, of course, but in the sense that what Jesus was must come to be present in us.
This is the deeper meaning of "faith." We must trust that God will do this because He has said He will. Our actions are guided not by a list of do-this-not-that but rather by the certain knowledge that He will transform us. That is the law of faith. It is the only law that we can keep. While we cannot do what is right in our own power, we can reach out by faith and trust that God will transform us. Our hope and desire for the law in the inward parts is made substantive through that faith.
"Now faith is the substance of things hoped for, the evidence of things not seen. For by it the elders obtained a good report." (Heb 11:1-2)