The Lord wants all his words to be spoken and none diminished. Jeremiah could have taken what God said and toned it down a little, maybe a little less harsh, to make it more acceptable in the people’s ears, but it would not have been acceptable to God. Words are important to God, not just the thought.
Jeremiah 26:
1 In the beginning of the reign of Jehoiakim the son of Josiah king of Judah came this word from the Lord, saying,
2 Thus saith the Lord; Stand in the court of the Lord's house, and speak unto all the cities of Judah, which come to worship in the Lord's house, all the words that I command thee to speak unto them; diminish not a word:
3 If so be they will hearken, and turn every man from his evil way, that I may repent me of the evil, which I purpose to do unto them because of the evil of their doings.