Q: Since unbelievers already have “the work of the law” written on the hearts (Rom. 2:14-15), what did God mean that He will put His law within believers and write it on their hearts (Jer. 31:33)?
罗马书 Romans 2:14-15
没有律法的外邦人，若顺着本性行律法上的事 (by nature do what the law requires)，他们虽然没有律法，自己就是自己的律法。这是显出律法的功用刻在他们心里 (They show that the work of the law is written on their hearts)，…
耶利米书 Jeremiah 31:33
“I will put my law within them (in their minds, in their inward parts), and I will write it on their hearts.”
以西结书 Ezekiel 36:26-27, also 11:19-20
“我也要赐给你们一个新心 (new heart)，将新灵 (new spirit) 放在你们里面，又从你们的肉体中除掉石心 (heart of stone)，赐给你们肉心 (heart of flesh)。我必将我的灵 (my Spirit) 放在你们里面，使你们 (cause you) 顺从我的律例，谨守遵行我的典章。”
Internalization of the law into the heart
“It means that under the New Covenant, the law will be internalized within believers to a degree that was not experienced by Israel under the Old Covenant (internalized in the sense that God’s people will know, love, and obey his law).” — Dr. James Anderson
“The two “writings” are not the same kind. (Remember, too, that “writing” is metaphorical in both cases!) The first “writing” is the general revelation of God’s moral law in the conscience. It’s what theologians often call “natural law.” But that has been distorted and corrupted by sin. The second “writing” refers to the renewing work of the Holy Spirit in regenerate believers, resulting not only in a clearer knowledge and understanding of God’s moral law, but also delighting in it and empowerment to obey it. Also, as I noted earlier, this is more accentuated under the New Covenant compared with the Old.” — Dr. James Anderson
The internalization of the law was God’s ideal for his people throughout OT history (e.g. Deut. 6:6; 10:16; 11:18; 30:6; Pss. 37:31; 119:34; Isa. 51:7) and was often obtained (Deut. 30:11-14; 2 Kings 23:25; 2 Chr. 31:21; Ps. 40:8; 119:11). Many times prior to Christ, it is apparent that the law of God regulated the lives of the people of Israel as little more than an external code. Obedience often came reluctantly and resulted from external pressures.
Already and Not Yet
The NT speaks this way because the promise of complete internalization of the law of God has begun within believers, but it has not yet been completed. There is a sense in which the hearts and minds of believers have been renewed by God’s grace (Rom. 12:1-2). At the same time, however, we are also commanded by NT writers to observe guidance from the Scriptures and to watch for corruption in our thinking (e.g. Rom. 1:18-2:29; Eph. 4:17-32; 2 Pet. 3:17). Everyone in the new creation after Christ’s return will have the law of God written on his or her heart. We will all love and delight in his ways, just as Christ already does (2 Cor. 3:16-18; 1 Thess. 3:11-13). In this sense, we expect Jeremiah’s prophecy to find complete fulfillment when Christ returns.