Jeremiah’s (ca. 600 B.C.) new covenant prophecy for Israel and Judah was, among other particulars, that the Torah would be written “on their hearts” (Jer. 31:33). The idea of the Torah within the people of Israel’s hearts was not novel– it goes back to Moses’ words in Deuteronomy (ca. 1405 B.C.):

“If you … keep His commandments and His statutes which are written in this BOOK OF THE LAW, …. But the word is very near you, in your mouth and IN YOUR HEART, that you may do it” Deuteronomy 30:10-14, emphasis mine (NKJV).

Likewise David wrote of the righteous person in his day (ca. 1000 B.C.) that …

“… the LAW of his God IS IN HIS HEART …” Psa. 37:31, emphasis mine — see also Psalm 40:8

Isaiah (ca. 740 B.C.) addressed the remnant in Israel with these words:

“Listen to me, you who know righteousness, the people IN WHOSE HEART IS MY LAW (torah ); Isa. 51:7a

We get more insight into what Jeremiah meant when we see how the idiom was used throughout the Tanakh. For instance Psalm 119:11 …

“I have stored up your word in my heart, that I might not sin against you.”

… shows us that before the new covenant was inaugurated it was incumbent upon the individual to store the Torah in his own heart, with the result that he would be less inclined to sin. The new covenant emphasizes God’s instrumentality in the placement process , but we may assume that the result would be something similar.

Isaiah demonstrates through synonymous parallelism that those in whose heart is God’s Torah are also those “who know [Heb. yada — i.e., have experiential knowledge of …] righteousness”

This word “righteousness” could either be pointing to what people do as a result of having the Torah in their hearts, or it could be suggesting a “righteousness” similar to how Paul uses the word in his letters, as in justification (i.e. “right-wising”). I’m inclined toward seeing the ideas as indivisible:

“you who have experiential knowledge of justification” (are the same as those who do Torah, are the same as those in whose heart is God’s Torah.”

Either way, Paul’s Galatian readers could expect this same experience with righteousness as Isaiah’s readers. The person whom David wrote about, with the Torah in his heart, had his moral steps made sure (Psa. 37:31). So could the new covenant believer. In Psalm 40:8 we see a psalm with messianic ramifications, but it still has application to Paul’s Galatian audience … and to us:

“I desire to do your will, O my God; your law is within my heart”.

This internalizing of the Torah was characterized by an urge to do God’s will, through His commandments. That is what the parallel structure of the above psalm suggests. Those under the new covenant can expect a similar desire to actualize what God requires of them. Through His commandments.

Under the old covenant the onus is on the individual and nation to place God’s Torah in their own hearts. Under the new covenant it is God’s responsibility. What the condemnation of the Law proved was that no one but God could successfully write God’s law on their heart. For New Covenant participants, however,

There is therefore now no condemnation for those who are in Messiah Yeshua. For the law of the Spirit of life has set you free in Messiah Yeshua from the law of sin and death. For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, in order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit. Romans 8:1-4

The “law of the Spirit of life” then is the same Torah as before, but now it is written on your heart, made a part of your nature. Therefore walk according to the Spirit-filled man, not according to your fleshly self.

Likewise, those “Old Testament” saints who had God’s Law in their hearts still found a copy of the “external” Law immensely valuable. The Torah inside them demonstrated where the written Torah stood in relation to their hierarchy of values.

It is clear from history and experience that present day believers also need the “external” Torah. A saint without the word of God is a dangerous person. He/she needs the stability that studying and applying the Law can bring–and the discipline it engenders.

That is one reason why Paul cannot be arguing against people in Galatia simply obeying the Torah. The idiom of “Law in your heart” does not discount the need for external regulations. It just demonstrates a person’s disposition toward obeying the one eternal Torah. Written or resurrected.

“Thus says the Lord GOD: Behold, I will take the people of Israel from the nations among which they have gone, and will gather them from all around, and bring them to their own land. And I will make them one nation in the land, on the mountains of Israel. And one king shall be king over them all, and they shall be no longer two nations, and no longer divided into two kingdoms. They shall not defile themselves anymore with their idols and their detestable things, or with any of their transgressions. But I will save them from all the backslidings in which they have sinned, and will cleanse them; and they shall be my people, and I will be their God. “My servant David shall be king over them, and they shall all have one shepherd. They shall walk in my rules and be careful to obey my statutes. They shall dwell in the land that I gave to my servant Jacob, where your fathers lived. They and their children and their children’s children shall dwell there forever, and David my servant shall be their prince forever. I will make a covenant of peace with them. It shall be an everlasting covenant with them. And I will set them in their land and multiply them, and will set my sanctuary in their midst forevermore. My dwelling place shall be with them, and I will be their God, and they shall be my people. Then the nations will know that I am the LORD who sanctifies Israel, when my sanctuary is in their midst forevermore.” Ezekiel 37:21-28