Matthew 12:40, commentary and pictures

Jonah's three days and three nights in the belly of the whale was a sign pointing to the Son of man being the same length of time in the heart of the earth. When the Pharisees and Sadducees demanded a sign from heaven, this sign of the prophet Jonah was the only sign to be given. No further outward proof was given because those asking needed spiritual renovation rather than intellectual enlightenment, which mere external evidence could not provide, even such a wonder as the sun standing still for Joshua. Sufficient attestation had already been given through Christ's own convicting words spoken with the power of the Holy Spirit and through signs direct from heaven, namely the angels' song to the shepherds, the star guiding the wise men, and the dove and voice at His baptism.

Just as Jonah's preaching was a sign to the Ninevites, so Christ's preaching was the sign to His generation. Since Nineveh repented at Jonah's warning, the Ninevites will rise in judgment against the generation that rejects One greater than Jonah, leaving those rejecters condemned.

The three days and three nights in the heart of the earth were foreshadowed when Jesus was separated from His parents after the Passover and was sought for three days. That childhood separation pointed to how His death for the sins of the world would separate Him from them for three days, after which He would reveal Himself and be found. Then their faith would rest on Him as Redeemer of fallen humanity and Advocate with the Father. That early experience prepared His parents for the sorrow of His death and for the hope of seeing Him again.