When Jonah disobeyed God in not going to Nineveh (Jonah 2), God had already prepared a great fish to swallow Jonah. Inside the great fish, Jonah acknowledges his disobedience and vows to turn around and do what God had asked him to do. When we SIN, we shouldn't just say we made a mistake. That would make us a MISTAKER, not a SINNER. When we sin, the only way to be free from the guilt we feel is to REPENT and TURN AROUND - to quit sinning with the help of God. If you are a mistaker, you will keep trying to get better. If you are a sinner, then you need a Savior.
Jonah teaches us some very important lessons about how God feels about the Lost. Those who don't know any better and continue to live in violence and misery. God loves them so much that He will send people to warn them. That is where we come in.