I will this once cause them to know... — The warning comes with all the emphasis of iteration, this once. As in a way without a parallel, once for all, they should learn that the name of the God they had rejected was Jehovah, the Eternal (Exodus 3:14), unchangeable in His righteousness. The thought is parallel to that of Ezekiel 12:15.
Related Commentaries of Jeremiah 16:21
Jeremiah 16:21
21 Therefore, behold, I will this once cause them to know, I will cause them to know mine hand and my might; and they shall know that my name is The LORD.d