Jeremiah 31:33-34

This shall be the covenant that I will make with the house of Israel That is, with those who are Israelites indeed, in whom is no guile, John 1:47, who are Jews inwardly, Romans 2:29, by the circumcision of the heart and spirit, spoken of and promised by God, Deuteronomy 30:6. I will put my law in their inward parts, &c. In the times of the gospel God's law is not abrogated and made void; for Christ came not to destroy the law, but to fulfil it: but it is written in the hearts of God's true Israel by the finger of his Spirit; and they become obedient to it from their secret approbation of it as holy, just, and good, and from the delight they take in it after the inward man. But it may be asked, How was this peculiar to this new covenant? Did not God of old write his law on the hearts of his people? Did not David and others, the servants of God, of whom we read in the Old Testament, serve God, out of a principle of love and delight in his law? We must answer, Undoubtedly they did, and the law of God was, in a measure, written in their hearts; but this was not through the virtue of the Mosaic dispensation, but through the grace of this new covenant, revealed and communicated, though but obscurely and partially, even under that dispensation. The principal design of the prophet here is evidently to express the difference between the law and the gospel: the law shows man his duty, the gospel brings the grace of regeneration, by which the heart of man is changed, and he is enabled to do his duty. All who, during the time of the Mosaic dispensation, attained salvation, were saved by virtue of this new covenant; but this was not then evidently exhibited; neither was the renewing grace of God so generally and largely given as it has been under the gospel. And they shall teach no more every man his neighbour This must not be so interpreted as if under the gospel there should be no more need of ministerial teaching, for Christ himself sent out his apostles to preach; nor yet as if there should be no further need of brethren in Christ teaching one another, for the contrary is commanded, Colossians 3:16. This expression only signifies the great increase of divine knowledge, especially of the knowledge of the being and attributes of the one living and true God, and of the relations in which he is pleased to stand to his people, which is the knowledge here chiefly intended. For they shall all know me, from the least unto the greatest Even the least of them, who have an interest in this new covenant, and are ingrafted into the good olive, and partake of the fatness of the root, even babes in Christ, and much more they who have arrived at the measure of the stature of his fulness; shall all savingly know me, and have eternal life in and by that knowledge. For I will forgive their iniquity, &c. Here God represents the free pardon of all their sins as being the root and foundation of this grace, and of all the privileges and blessings of this new covenant.