And all of them from the greatest to the least put on sack-cloth. I love that verse. This does not say they believed in Jonah. The goal of a preacher is not to have people believe in him. Too many preachers today are out of God's will by preaching their own messages.
Preachers ought to preach the word.
First I am not smart enough to preach anything else. If I preach on politics a lot of you know more about that than I do. Same with social issues. Secondly, I am smart enough to know that God will richly bless if His word is preached. Faith comes by hearing.