"Then some of the scribes and pharisees said to him, "Teacher, we wish to see a sign from you." He said to them in reply, "An evil and unfaithful generation seeks a sign, but no sign will be given it except the Sign of Jonah the prophet. Just as Jonah was in the belly of the whale three days and three nights, so will the Son of Man be in the heart of the earth three days and three nights."  (Matthew 12: 38-40).

      The scribes and pharisees did not believe that the miracles Jesus performed was proof of His being sent from heaven. Some of the scribes and pharisees thought Jesus performed these miracles by the power of satan. So they demanded a sign from Jesus. In essence they said to Jesus; Look Jesus, the people are believing in you, but we are too smart, so give us a sign, but make it a good one, a real good one, so there can be no doubt, and then we too will believe in you. Jesus did not point to any prior miracles; the healings, the exorcisms as proof of his being sent from heaven. Jesus told them "No sign shall be given, except, the Sign of Jonah". So a sign will be given, only one sign, as proof to an unfaithful generation.
         Jesus was talking about his death and resurrection. As Jonah was in the belly of the whale (or big fish) for three days and three nights; so shall the Son of Man be in the heart of the earth. His death and resurrection are the event; but the Shroud is the sign of that event. The Shroud is the Sign of Jonah. The sign given to the whole world as proof that Jesus is the Son of God come down from heaven.

      A sign is something that points or directs to, or symbolizes, or gives more information than itself. If we come across a one-way sign, we know to go only one way. It also relays more information; we don't expect traffic to come from the opposite direction. Driving in the opposite direction will be wrong, and we can possibly get into an accident.
         The Shroud is the sign or picture which shows the correlation between Jonah and Jesus.
         Jonah was thrown overboard and swallowed by a whale (or big fish). Jonah died in the belly of the whale. There is much debate about whether or not Jonah was indeed dead or alive. (Jonah 2:3) states; "from the midst of the netherworld (Hades, Sheol) I cried for help".
         The Old and New Testament both affirm that the soul of a dead person is in Sheol.
   (Old Testament - Psalms 16:10) "Because you will not abandon my soul to the netherworld, nor will you suffer your faithful one to undergo corruption."
   (New Testament - Acts 2:27) "Because you will not abandon my soul to the netherworld, nor will you suffer your Holy One to see corruption".

  Jonah said, " Weeds were wrapped around my head" ------------------- Jesus wore a crown of thorns on His head.
  Jonah died --------------------------------------------------------------------------- Jesus died
  Jonah's body  was in the belly of the whale 3 days ------------------------ Jesus' body  was in the sepulchre for 3 days.
  Jonah's soul was in Sheol ------------------------------------------------------- Jesus' soul was in Sheol.
  Jonah was revived 3 days later ------------------------------------------------- Jesus resurrected 3 days later.

         The Shroud tells this story, it shows the whole Triduum,. Jesus' Passion, Death and Resurrection. We know that Jesus Resurrected before 4 days, as there are no signs of putrefaction on the image of the Shroud. There are signs, however,  of what Jesus endured. The marks of His Passion and Crucifixion. Jonah also would have shown the marks on his body of his ordeal. From the stomach acid of the whale, Jonah probably did not have any hair left on his body. His skin would have been bleached pure white and wrinkled.
   The story of Jonah points or leads to Jesus. Our Good and Loving God gives us this precious relic; The Shroud, The Sign Of Jonah, to help us. To help reveal Jesus to us.