Matthew Poole and continuators on Jeremiah 29:4

4 Thus saith the LORD of hosts the God of Israel, unto all that are carried away captives, whom I have caused to be carried away from Jerusalem to Babylon .

So as this Letter was not wrote from himself advising them charitably, but he had commission from God, by whom he mindeth them, as the principal efficient cause they were ordered to be carried away, though their own sins were the meritorious cause▪ and Nebuchadnezzar with his Captains and Souldiers were the instrumental cause.