What sin did Jesus say would never be forgiven?

Blasphemy against the Holy Ghost

And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come. Matthew 12:31-32

(See also Luke 12:10 and Mark 3:29)