Jonah has a special place in Jesus' teaching ministry.  He is mentioned in eight verses of the Gospels in connection with his miraculous deliverance from the belly of the great fish after three days (the first ‘sign of Jonah’ which prophesied the resurrection) and his preaching (the second sign prophesying the potency of the gospel to those who believe and repent).

In these verses, Jesus was focussing on the people who heard and believed Jonah's preaching - that God was angry with their sin.  They realised that God was fully just in bringing ruin upon them, but that He might be merciful if they stopped their wicked ways and were genuinely sorry (Jonah 3:5-10).  And, of course, God was merciful to the repentant sinners.  In the same way, the pagan Queen, probably from the Horn of Africa travelled a great distance to hear God’s wisdom from Solomon and was willing to give her wealth to honour God’s servant (1 Kings 10:1-13)

Jesus was drawing a contrast between the pagans who believed God's Word and His hearers, Jews who should have believed that what He said was from God but did not.  The religious people could not make Him out: they asked for miraculous signs but could not interpret all the signs He had already given, showing that He was the Son of God (John 14:11).  Jesus commented that the pagans of Nineveh, and the gentile Queen of Sheba, would stand with God’s approval at the judgement.  Their example would shame those who claimed to be the people of God, but who would be condemned - because of their wickedness and lack of faith in God's Saviour.

Repentance is a critical factor in receiving God's mercy (Acts 2:38).  It is the only God-approved response to being aware that we are in the wrong.  It may be that we already know what is right and wrong, and our conscience is fired by our sinfulness.  But often we have seared our consciences by repeated sin (1 Timothy 4:2).  It is not until we hear God's Word proclaimed (in our Bible reading, church sermons, or counsel from another believer) that we realise how bad our sinfulness is, and how much it spoils our relationship with God.  When we wake up to the problem, we need to repent, immediately and wholeheartedly.  If the Ninevites could do it, so can we.  Indeed, we must!