Albert Barnes Commentary Jeremiah 15:19

"Therefore thus saith Jehovah, If thou return, then will I bring thee again, that thou mayest stand before me; and if thou take forth the precious from the vile, thou shalt be as my mouth: they shall return unto thee, but thou shalt not return unto them." — Jeremiah 15:19 (ASV)

Jeremiah had questioned God’s righteousness (see Jeremiah 12:1 note); he is told, If you return, if you repent of your doubts, and think only of your duty, then will I bring you again, then will I cause you again to stand before Me. To stand before a person means to be his chief officer or vicegerent. It implies therefore the restoration of Jeremiah to the prophetic office.

If you take forth the precious from the vile - i. e., if you cause the precious metal to come forth from the dross. Jeremiah was to separate in himself what was divine and holy from the dross of human passion. Let him abandon this mistrust, this sensitiveness, this idea that God did not deal righteously with him, and then he shall be as God’s mouth, i. e., as the organ by which God speaks.

Let them return ... - Rather, they shall return to you, but you shall not return to them. A flattering prophet perishes with the people whom his soft speeches have confirmed in their sin: but the truthful speaking of God’s word saves both.