Dora Bas Rivka Silver O'H
G-d spoke to Jeremiah during the reign of King Yehoyakim of Judah, making this a decade or two earlier than the prophecy recorded in the previous chapter. (They're not in chronological order.) G-d told Jeremiah to go to the Rechavim (also known as the Kenites), who were descendants of Yisro (Jethro), the father-in-law of Moses. They were to be brought to the Temple and given wine to drink. So Jeremiah brought the whole family to the Temple and gave them cups of wine. He told them to drink up, but they replied that they don't drink. Their ancestor, Yonadav the son of Rechav, commanded his descendants to abstain from wine forever. They were also instructed not to build houses or plant fields, but to live in tents. (The Kenites were not landowners and were able to move about. They apparently eschewed city life.) And so, they heed the words of their ancestor. However, when Nebuchadnezzar first invaded, the Kenites migrated to Jerusalem for refuge and had no alternative but to live in houses in the city.
G-d told Jeremiah to say to the people of Judah and Jerusalem that they should learn a lesson from the Kenites. They listen to their ancestor's words and abstain from wine. Yonadav was a human and hasn't spoken to his descendants since he died. G-d is G-d and He sent constant reminders through the prophets saying that the Jews should give up their evil ways and return to Him, but they still won't listen! So, all the catastrophes G-d warned about will come to pass, but the Kenites will be rewarded. Because they were careful to follow their ancestor's instructions, G-d will see to it that there will always be members of their family serving him. (According to the Midrash, cited by the Radak, they were great rabbis and members of the Sanhedrin. Additionally, many of the daughters married Kohanim and had sons who served in the Temple.)