Jer. 31:31-34; 2 Cor. 3:3; Heb. 12:24.
Please note the contrast between the Old and the New Covenant. The Old with its "thou shalt" and "thou shalt not," and the New with its "I will." Demand in the Old; gift or grace in the New. While it may be seen that the New Covenant will yet be made with the house of Israel and with the house of Judah, in its literal terms, we would point out that God is acting today on the basis and in the spirit of it. So, in the institution of the Supper the Lord Jesus said, "This cup is the New Covenant in My blood."
The blessings which are ours now anticipate, in principle, those of Israel in that day when they shall enjoy the actual terms of the New Covenant as propounded by Jeremiah.
If theirs is to be a writing in the heart so there is a writing in our hearts now. The sovereign mercy and operation of God are as necessary for us as for them. Just as the law proved them to be utterly incorrigible, so we learn that "in me (us) good does not dwell" and, on the contrary, "Sin dwells in me (us)" (Rom. 7:17, 20). Our hearts, then, are not less stony than theirs. To be rendered impressionable God must operate. This He has done in new birth and thus, in new Covenant mercy, He has given us "fleshy tables" "Of His own will begat He us" (James 1:18). "According to His abundant mercy has begotten us again … " (1 Peter 1:3).
He is not writing the law today — He is writing Christ. Giving impressions of Christ and forming Him in us by the Spirit of the living God.
In Ezek. 36:27, in addition to the heart of flesh, He says, "I will put My Spirit within you, and cause you to walk in My ways …" We have the Spirit now as the Spirit of His Son, sent into our hearts crying, "Abba, Father," and as the bond of union with Christ in the glory, and with one another. Yet, incidentally, the righteous requirements of the law are fulfilled in us who walk not according to flesh but according to the Spirit.
He shall be their God, He is ours as the God and Father of our Lord Jesus Christ. He is our God as we are in relationship with Him as sons, being associated with the Son of God. We are also His people as the apostle Peter teaches in 1 Peter 2:10. James in his summing up in the crisis of Acts 15, refers to Peter's declaration of how God had visited the nations to take out of them a people for His Name. Israel, now Lo-Ammi — not My people — will be Ammi, in new Covenant blessing (cf. Hosea 1 and 2). They will then be set up with a national polity of their own on earth yet, nonetheless, distinct from the Gentiles. The people of God are completely taken out today, from Jew and Gentile, "our citizenship is in heaven."
The development of this thought will take us on to the truth of the one body — not as Jew nor as Gentile but — as an entirely new, separate, and heavenly entity, in which there is a completely new man devoid of all racial, religious, social and secular differences, "but Christ is everything and in all." See for this Colossians 3:10-11. They shall then have no need that any should teach them, "Know Jehovah, for all shall know me" (Heb. 8:11). Ours, though, is to be in "the full knowledge of Him."
Divine and eternal relationships with the affections proper to them are in the realm of this knowledge — God is revealed, Father, Son and Holy Spirit. Their sins shall all be forgiven them and they shall be at perfect rest in their knowledge of God. We are also forgiven and justified. As transferred from Adam to Christ we are in the gain of Romans 5:17-19, for instance. We know the blessedness of the favour of our God; the gift of righteousness also with the hope of reigning in life by the one, Jesus Christ; with the free gift which has led to justification, not only from offences, but to life in our risen racial Head. The grace of our blessing is delineated in the risen Christ. We see Him, "raised from the dead by the glory of the Father" clear from everything to which He had died — and we in Him. Deliverance from sin's dominion, as also from the curse of a broken law. All then demonstrating to us the greatness of the blessings which are ours — not being under the literal terms of the New Covenant, but as being blessed in the spirit of it.
In conclusion, our blessings are great indeed, but our hearts are drawn out to Him in Whom this disposition of God in New Covenant mercy has been made known. Affection for Christ with appreciation of His redeeming work will help to keep us right in this evil day. For we are come "to Jesus, the Mediator of the New Covenant, and to the blood of sprinkling, that speaks better things than that of Abel. See that ye refuse not Him that speaks. For if they escaped not who refused Him that spake on earth, much more shall not we escape, if we turn away from Him that speaks from heaven" (Heb. 12:24-25).