They might reasonably suspect that the Chaldeans would have a jealous eye upon any conflux of people to Jerusalem, especially military men, and therefore be something suspicious of him who was the Chaldean deputy governor; the securing of them from fears on this account was the cause of this voluntary oath taken by Gedaliah. He encourageth them to be servants to the Chaldeans, and to dwell in the land, assuring them that if they would, they should fare well; he was well enough assured of the Chaldeans favour, that if they would live peaceably in their own land, they should, they would not come any more to carry them away captive.
Related Commentaries of Jeremiah 40:9
Jeremiah 40:9
9 And Gedaliah the son of Ahikam the son of Shaphan sware unto them and to their men, saying, Fear not to serve the Chaldeans: dwell in the land, and serve the king of Babylon, and it shall be well with you.