Unbelief is a "cause", not a symptom. The Pharisees try to trap Jesus by demanding on their terms a sign. Jesus, in turn, tells them that it is an evil and adulterous generation that demands a spectacle. They will not be given a sign by their demands but out of the mercies of God, they will be given a sign: Just as Jonah was in the belly of the great fish, and came out alive, so Jesus will be in the earth, dead, and come out alive!