The relevant scripture is Doctrine and Covenants 1:4-5:
4 And the voice of warning shall be unto all people, by the mouths of my disciples, whom I have chosen in these last days.Nibley pointed out that the scripture means that we are to serve as a voice of warning and the only thing stopping us is ourselves.
5 And they shall go forth and none shall stay them, for I the Lord have commanded them.
We usually are pretty effective at stopping ourselves. We might stumble because of the placing of our words (Ether 12:25). We might be worried because of the mocking and the pointing of fingers from the great and spacious building. We can come up with all sorts of excuses for not providing a warning voice.
Of all people, Jeremiah had plenty of reasons not to issue that voice of warning. He was mocked and imprisoned and physically abused and at one point was ready to give it up:
7 O Lord, thou hast deceived me, and I was deceived: thou art stronger than I, and hast prevailed: I am in derision daily, every one mocketh me.When we have the truth as a burning fire shut up in our bones, we cannot stay it, and none can stay us.
8 For since I spake, I cried out, I cried violence and spoil; because the word of the Lord was made a reproach unto me, and a derision, daily.
9 Then I said, I will not make mention of him, nor speak any more in his name. But his word was in mine heart as a burning fire shut up in my bones, and I was weary with forbearing, and I could not stay. (Jeremiah 20:7–10)