After all the miracles Jesus has done, the Pharisees and Sadducees ask for a miracle to prove that Jesus is from God. The Sadducees were a rival group of the Pharisees, but this time they are teaming up to oppose Jesus. Jesus tells his disciples they will see the sign of Jonah. Jesus had said this once before in chapter 12 with more explanation there. The story of Jonah was told in the Old Testament. He had been thrown off a ship and swallowed by a fish. After three days, the fish spit him out on the shoreline. Jesus is predicting he will be dead for three days and rise from the dead. We won’t see this till later, but most Pharisees and Sadducees will reject that sign too. Jesus asks his disciples who they think he is. Peter correctly answers that Jesus is the Christ or Messiah. Jesus then begins to move toward his death.
Perhaps you have talked with someone who has said that if they could witness one of those miracles, like the ones described in the Bible, they would believe in Jesus. The people who did see those miracles didn’t all believe in Jesus, so more miracles aren’t the answer. We can believe in God from what we can see in creation and changed lives, but it still takes faith to trust in a God we cannot see.