When we think of Jonah and Jesus and what they have in common, the first thing that usually comes to mind is that Jonah was in the whale (or "big fish" – take your pick) for 3 days and Jesus was in the tomb for 3 days. However, as Jesus points out in the Gospel, the main comparison to be made between him and Jonah is the announcement of the coming of God’s judgement.
The sign to the Ninevites that Jesus makes reference to was not Jonah’s time in the whale – they probably weren’t even aware of it at the time. Rather Jonah himself was the sign – a sign of warning and a call to repentence. This is what Jesus was telling the people of Jerusalem in his time. It is also what he is telling us we should be to the world in our own time.
"All it takes for evil to triumph is for good men to do nothing." Had Jonah been successful in evading God (as if that were possible), then Ninevah would have been destroyed, on account of the evil men in it. However, due to the actions of one good (albeit reluctant) man, an entire city was converted from evil.
Are we willing to speak out against economic injustice, or do we let the poor suffer, rationalizing it as their choice to be poor and suffer for it? Are we willing to speak out against abortion, or do we rationalize it as a woman’s choice to kill her child and God will handle it? Are we willing to speak out against all sins, or just those we are comfortable condemning, while keeping silent about those that are "fashionable"? "Men do not differ much about what things they will call evils; they differ enormously about what evils they will call excusable." (G. K. Chesterton, Illustrated London News, 10/23/1909)