Charles Ellicott Commentary Jeremiah 31:34

"and they shall teach no more every man his neighbor, and every man his brother, saying, Know Jehovah; for they shall all know me, from the least of them unto the greatest of them, saith Jehovah: for I will forgive their iniquity, and their sin will I remember no more." — Jeremiah 31:34 (ASV)

They shall teach no more every man his neighbour ... —We trace in that hope for the future the profound sense of failure which oppressed the mind of the prophet, as it has oppressed the minds of many true teachers since. What good had come from all the system of ritual and teaching which the Law of Israel had provided so abundantly? Those repeated exhortations by preachers and prophets that people should “know the Lord”—what did they present but the dreary monotony of an “old worm-eaten homily”?

To know Him, as He truly is, required nothing less than a special revelation of His presence to each person’s heart and spirit. That revelation was now, for the prophet's comfort, promised to all who were willing to receive it as the special gift of the near or distant future, which opened to his view in his vision of a restored Israel. Here also the words of Jeremiah echo those of an older prophet (Isaiah 54:13) and find their fulfillment in those of Christ (John 6:45).

I will forgive their iniquity ... —The second clause repeats the promise of the first, in a form that is, perhaps, from the necessity of the case, speaking in human terms. Our thoughts of God as All-knowing preclude the idea of any limitation of His knowledge, such as the words I will remember no more imply. What is meant is that He will be to the one who repents and knows Him as He truly is, in His essential righteousness and love, as people are to other people when they “forget and forgive.” He will treat the past offenses, even though their inevitable consequences may continue, as though they had never been, as far as they affect the soul’s communion with God. He will, in the language of another prophet, blot out the sins that still belong to the indelible and irrevocable past (Isaiah 43:25; Isaiah 44:22).