Just last week we traversed the Book of the Prophet Jonah. Today's Gospel finds Jesus referencing the "sign of Jonah" in his dispute with the crowd.
Oftentimes, when the question of the "sign of Jonah" comes up for discussion, people will remember that just as Jonah spent three days in the belly of the great fish (whale) before he was spewed up on the seashore, Jesus spent three days in the bowels of the earth before his resurrection.
However, there is another way to look at this issue; namely, Jonah's success as a prophet. You will remember that Jonah went but a day's walk through the great city of Nineveh before the people of that city, including its king, repented of their sin and did penance thereby forestalling God's punishment. Another way to look at the "sign of Jonah" might be this very fact; Jonah's preaching was effective in bring the sinners to repentance. There is no question that this is also true of Jesus' preaching. Throughout his ministry in Galilee and Judea, Jesus confronts people who turn away from their former ways of life. Chapter fifteen of Luke's Gospel is filled with parables about God seeking out the sinner. Jesus has also interacted with non-Jews and will continue to do so. Finally, it is in St. Luke's Gospel that Jesus extends his forgiveness to one of the thieves who is crucified with him and promises that he will be with him in paradise.
For a prophet, what could be more potent sign of effectiveness than the conversion of the sinner?