Charles Ellicott 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)

Therefore thus says the Lord ... —The Divine voice within answers the passionate complaint. The prophet also needs, no less than the people, to return to his true mind, to repent of his murmurings and distrust. Upon that condition only can he again stand before the Lord in the full sense of that word, and minister to Him as a prophet-priest (compare 1 Kings 17:1; 1 Kings 18:15; 2 Kings 3:14).

He has to distinguish between the precious and the vile, between the gold and the dross, between a righteous zeal and the despondent bitterness which is its spurious counterfeit, not only in the people to whom he speaks but also in himself. Above all he must beware of being tempted by his sense of failure, to return to the people in the manner of one who tunes his voice according to the time. Rather, they must return to him and rise to his level, both returning to Jehovah.