The old saying, "Seeing is believing" is the start. Four ghosts visit him...Marley, Scrooge's old business partner, is the first. When Scrooge sees how Marley's ghost is weighed down by chains and the symbols of their business together, and Marley tells him how he must wander for eternity and that Scrooge's chain will be twice as long and heavy as Marley's, Scrooge begins to pay attention. It is not until he travels back in time to Christmas Past, visits Christmas Present and Christmas Future that Scrooge not only sees, but feels. Those feelings of being loved then abandoned were true to him. The feeling of joy he feels toward his little sister, his one-time fiancee, and his old boss, Mr. Fezziwig reminds him of how life used to be and could be again. The pain he senses in his nephew's house and in the Cratchit household because of Tiny Tim also makes a dent in his icy demeanor. Scrooge winces when the ghosts use his own words against him at crucial times in the play, and he realizes that he could change things so they wouldn't be quite so horrible for not only himself but for his family and the family of his employee, Bob Cratchit. It is with this newfound knowledge and sense of humanity that he awakens after the third ghost has left to rejoice that he hadn't missed Christmas. He begins spending a little money and spreading happiness...changing the future for many.
A Christmas Carol is a story of Scrooges' transformation. The device used were the visits by the spirits to show him what had happened to him and what choices he made when young, then how those choices were currently affecting him and those around him, and finally where those choices were leading him. As he realized that changing his choices would be for everyone's best interest, including his own, he begain the transformation.