Scrooge was coming home at night. As he looked at the door knocker, he suddenly saw the face of Marley, his long-deceased business partner. This happened without any "intermediate process of change." Suddenly Marley's face just appeared. The face was not angry, but looked to Scrooge exactly as he had in real life, with his glasses upon his forehead. The hair moved as though there were some sort of a breeze, and the open eyes did not move at all. There was something about the knocker that Scrooge found to be scary, although that was just a feeling and not a result of the face actually looking scary. As he stood looking at the face, suddenly it became a knocker again.
In this instance, Ebenezer Scrooge returned home to a house previously belonging to his erstwhile and now-deceased partner, Jacob Marley. Marley had died seven years ago and had much the same qualities as Scrooge. He had been uncaring, selfish, and greedy, just like Scrooge. When Scrooge wanted to turn the key in the front door lock, his attention was drawn to the large knocker on the door. It had undergone a supernatural transformation and looked like the face of his dead partner, as the narrator explains in Stave 1 of the chapter titled "Marley's Ghost."
And then let any man explain to me, if he can, how it
happened that Scrooge, having his key in the lock of the door,
saw in the knocker, without its undergoing any intermediate
process of change -- not a knocker, but Marley's face.
Marley's face. It was not in impenetrable shadow
as the other objects in the yard were, but had a
dismal light about it, like a bad lobster in a dark
cellar. It was not angry or ferocious, but looked
at Scrooge as Marley used to look: with ghostly
spectacles turned up on its ghostly forehead. The
hair was curiously stirred, as if by breath or hot air;
and, though the eyes were wide open, they were perfectly
motionless. That, and its livid colour, made it
horrible; but its horror seemed to be in spite of the
face and beyond its control, rather than a part or
its own expression.
This appearance is a precursor to Marley's later appearance when he admonishes and warns Scrooge about his attitude. Marley had been condemned to walk the earth for eternity because of his uncaring and selfish manner. He has come to warn Scrooge about his lot and wishes him to avoid suffering the same fate. He informs Scrooge that he will be visited by three spirits and tells him that he should listen to them or otherwise expect to suffer eternal damnation.
Fortunately, Scrooge does as he is told and is saved. He becomes the embodiment of the spirit of Christmas through his generosity, kindness, and compassion.