Those who have believed are the gospelizers of verse 15 and the gospelized of verse 16. The danger of signs. Sign seeking is offensive to God. But none will be given it except the sign of the prophet Jonah. Signs in themselves prove nothing about the individual.
Signs have nothing to do with the messenger.
The disciples were having trouble believing that Jesus was alive. She went and reported to those who had been with Him, while they were mourning and weeping. When they heard that He was alive and had been seen by her, they refused to believe it . After that, He appeared in a different form to two of them while they were walking along on their way to the country. They went away and reported it to the others, but they did not believe them either .