Why will people no longer need to teach each other to know the LORD in the new covenant?
In the new covenant, God writes His Law in their hearts and inward parts, so knowledge of the LORD becomes inward and automatic rather than taught externally. They shall all know Him, from least to greatest, because He forgives their iniquity and remembers their sin no more.
Les explains this is the New Covenant made with the house of Israel and the house of Judah, not according to the covenant God made with their fathers when He brought them out of Egypt. God says He will put His Law in their inward parts and write it in their hearts; He will be their God, and they shall be His people. Then verse 34 says they shall teach no more every man his neighbor and every man his brother, saying, “Know the LORD,” for they shall all know Him. Les emphasizes this will be without benefit of any instruction; it will be an automatic thing that they know He is God, from the least unto the greatest, because He will forgive their iniquity and remember their sin no more. Les connects this to Zechariah’s promise that a surviving remnant—one-third—will suddenly realize Jehovah is their God and recognize Jesus of Nazareth as their Messiah, and God will call them My people once again. He does not call them that today, but He will then. This is Israel’s future, but only a remnant; the rest will be cut off and die.
And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more.