At the time in which the Canterbury Tales are set, the Roman Catholic Church was a dominant factor in English society. The archdeacon featured in the Friar's Tale used a particularly repulsive individual as a summoner, the individual in charge of delivering notices charging people with sinning. These persons could avoid being publicly charged with their sins if they adequately paid the summoner for dismissal of the charge. This is all based on actual practice in medieval society.
Superstition was widespread and unquestioned. Belief that the bailiff was a devil and could change form as he wishes would be readily accepted. The idea that the summoner could be snatched away to hell based on the widow's curse would be considered completely believable.