Baptist Believer's question is a fair one to discuss.
I went back and re-read the book of Jonah. Jonah was a reluctant preacher, as we know. When God finally got his attention, he gave Jonah a specific message to preach.
“Go to the great city of Nineveh and proclaim to it the message I give you.” 3 Jonah obeyed the word of the Lord and went to Nineveh. Now Nineveh was a very large city; it took three days to go through it. 4 Jonah began by going a day’s journey into the city, proclaiming, “Forty more days and Nineveh will be overthrown.” 5 The Ninevites believed God. A fast was proclaimed, and all of them, from the greatest to the least, put on sackcloth.
Later, in light of Nineveh's repentance, God "relented" and did not destroy the city. And remember, even that irritated Jonah. He wanted God to zap 'em all. Jonah was not the least bit happy that God did not destroy Nineveh.
To me, God used Jonah's preaching to effect a change in the people who heard it. Jonah was a reluctant prophet, but he wasn't a false prophet. He preached exactly what God told him to preach.