What sign did Jesus give as proof He was the Messiah in Matthew 12?
The sign Jesus gave was the sign of the prophet Jonah: as Jonah was three days and three nights in the whale's belly, so the Son of man would be three days and three nights in the heart of the earth. No other sign was given to that evil and adulterous generation.
Les teaches from Matthew 12:38-40 that when they asked to see a sign, Jesus answered that an evil and adulterous generation seeks after a sign, and no sign would be given except the sign of the prophet Jonah. Les explains that just as Jonah was three days and three nights in the whale's belly, so the Son of man would be three days and three nights in the heart of the earth. In the surrounding context, Les connects this downward direction to the Old Testament economy, where paradise was down in the center of the earth until after Christ's resurrection, because Christ's atoning blood had not yet been shed and animal blood only covered sin.
Then certain of the scribes and of the Pharisees answered, saying, Master, we would see a sign from thee.
But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas:
For as Jonas was three days and three nights in the whale’s belly; so shall the Son of man be three days and three nights in the heart of the earth.