What does the 'sign of Jonah' mean in the Bible?

The 'sign of Jonah' mentioned in Matthew 12:40 highlights the parallel between Jonah's three days in the belly of the fish and Christ's three days in the heart of the earth. This comparison serves to signify the sacrificial nature of Christ's mission, illustrating that just as Jonah was delivered from the fish, so too would Jesus be raised from the dead. This sign underscores the necessity of repentance and faith in Christ, emphasizing that His death and resurrection are pivotal to the gospel message—salvation for those who believe due to God's sovereign grace.