Albert Barnes Commentary Jeremiah 6:10

"To whom shall I speak and testify, that they may hear? behold, their ear is uncircumcised, and they cannot hearken: behold, the word of Jehovah is become unto them a reproach; they have no delight in it." — Jeremiah 6:10 (ASV)

Give warning - Rather testify.

Reproach - They make the Word of God the object of their ridicule.