First off, the point you’re interested in is actually Jeremiah 31:34.
Here’s a more literal translation of it:
“And they shall teach no more every man his neighbor, and every man his brother, saying: ‘Know the Lord,’ for all shall know Me, from the least of them even to the greatest,” says the Lord. “For I will forgive their iniquity, and I will remember their sin no more.”
Jeremiah 31 is talking about the people coming back from Babylon. So in this case, every Jew will personally know what the Lord has done, because they’ll have had their collective sins as a people forgiven and they’ll have been brought back to the Promised Land. (That’s why it says earlier that a man will only die for his own iniquity.)
There are some bits talking about the Messianic stuff to come (“A woman shall compass a man”, which is about Mary giving birth to Jesus), but mostly it’s about coming back from Babylon.
Now, in the future when Jesus comes again, we also won’t have to tell each other to know the Lord, because He’ll be right there and we’ll all know all about Him. He will bring us home to His Home, so Jeremiah 31 does have some future application as well as the historical and prophetic literal reading.
But that time is not yet, so we do have to talk to our neighbors and our brethren about the Lord.