The Preacher has told us why the new covenant is superior to the old; namely, because it is founded upon the superior Priest.
But now he tells us of another reason which comes right out of the Prophet Jeremiah (31:31-34). Actually, it’s about another day: Jeremiah prophesied of a new day when God would establish a new covenant. But this implies that there was something wrong with the old covenant, some fault. Now we must be careful here. Paul tells us in Romans 7:7-25 that the law is good, spiritual, and would have worked life in us were it not for our sinful natures. Granted, the new is better than the old, but the fault lies with sinners who could not live up to the old. So, the Lord says in Jeremiah, “They did not continue in my covenant.”
But God now proves his grace and mercy by providing a new and better covenant founded on better promises. And what is that promise upon which the new covenant is founded? Why, salvation by grace through faith in the person and work of Jesus Christ! But it even goes further than that. The primary fault with the old covenant, or the reason why sinners could not live up to it, is that sinners must be changed into saints before they can ever hope to love and obey God. There must be a transformation. And this is solely the work of the Holy Spirit sent by the Father and the Son to regenerate sinful men. The Holy Spirit does this through convicting one of sin and convincing him of his need for Christ, the acceptance of which leads to that new birth and new life.
And this is exactly what Jeremiah describes: “I will put my laws into their minds, and write them on their hearts, and I will be their God, and they shall be my people.” The one born of the Spirit has the Spirit within who reminds the believer of God’s word. And that word, that law, is no longer command but life. The believer’s love for God comes from outside of him into him through his rebirth. It is not natural to him. But now that he is born again, born from above, with God’s Spirit within him, the believer’s mind, will, and affections are changed. Furthermore, no one has to say to him, “Know the Lord”; he knows the Lord, as do millions of others born of the Spirit. Under the old covenant, the law was ever outside him; now it is within him. The Spirit has changed him, and continues to change him, from one degree of glory to another. So, the new covenant is founded upon better promises, the promised coming of the Holy Spirit upon his people (Luke 24:49; Acts 2).