Charles Ellicott Commentary Jeremiah 26:15

"Only know ye for certain that, if ye put me to death, ye will bring innocent blood upon yourselves, and upon this city, and upon the inhabitants thereof; for of a truth Jehovah hath sent me unto you to speak all these words in your ears." — Jeremiah 26:15 (ASV)

But know you for certain. —Literally, with the Hebrew emphasis of reduplication, Knowing, know you. The appeal is addressed, it will be remembered, to the lay judges, the princes, and the people, not to the priests and prophets who accused him. He believes that they at least would shrink from shedding innocent blood. And he solemnly protests that he is innocent of any wilful attack upon what his countrymen revered. He has spoken, but it has been by a constraint above his own will. A necessity has been laid upon him (1 Corinthians 9:16).