An Evocation of Hades

"Hear me!" cried the Ghost.  "My time is nearly gone."

"I will," said Scrooge.  "But don't be hard upon me!  Don't be flowery, Jacob!  Pray!"

"How it is that I appear before you in a shape that you can see, I may not tell. I have sat invisible beside you many and many a day."

[ Because Scrooge, by sitting alone in the dark in a bare, cavernous, hollow, empty house is unwittingly performing a ritual invocation of Pluto, by recreating the conditions and the environment of the throneroom of the King of the Underworld in Hades (which, unlike Christian Hell is always cold, not hot.). 

Thus, he magickally gains the ability to see and talk to dead people via a ritual invocation of Hades. Without having any notion that he might be doing anything of the sort.

Marley, as the wraith of a Christian soul is forbidden from explaining this to him, since the invocation of Pagan Deities is taboo and a forbidden practice, not to ever br discussed or spoken-of openly, even to the invoker who does not actually realise that they are doing it. ]

The "Devil" in The Exorcist is explicitly identified in both the novel and the film as being Pazuzu, ancient Sumerian God of the South-West Wind - remember, at this point (and still) gods could either be worshipped, or they could be bought off and appeased (which amounts to the same thing - you acknowledge and recognise their power over a particular area of nature.

Pazuzu was a god more to be bought off rather than worshipped - why?

Because the South-West Wind carried swarms of Locusts over from East Africa to decimate the crops of Sumer.

But he isn't "Evil", as such - just associated with bad THINGS.

It was not an agreeable idea.  Scrooge shivered, and wiped the perspiration from his brow.

"That is no light part of my penance," pursued the Ghost.  "I am here to-night to warn you, that you have yet a chance and hope of escaping my fate.  A chance and hope of my procuring, Ebenezer."

The apparition walked backward from him; and at every step it took, the window raised itself a little, so that when the spectre reached it, it was wide open.  It beckoned Scrooge to approach, which he did.  When they were within two paces of each other, Marley's Ghost held up its hand, warning him to come no nearer.  Scrooge stopped.

Not so much in obedience, as in surprise and fear: for on the raising of the hand, he became sensible of confused noises in the air; incoherent sounds of lamentation and regret; wailings inexpressibly sorrowful and self-accusatory.  The spectre, after listening for a moment, joined in the mournful dirge; and floated out upon the bleak, dark night.

Scrooge followed to the window: desperate in his curiosity.  He looked out.

The air was filled with phantoms, wandering hither and thither in restless haste, and moaning as they went.  Every one of them wore chains like Marley's Ghost; some few (they might be guilty governments) were linked together; none were free.  Many had been personally known to Scrooge in their lives.  He had been quite familiar with one old ghost, in a white waistcoat, with a monstrous iron safe attached to its ankle, who cried piteously at being unable to assist a wretched woman with an infant, whom it saw below, upon a door-step.  The misery with them all was, clearly, that they sought to interfere, for good, in human matters, and had lost the power for ever.

Whether these creatures faded into mist, or mist enshrouded them, he could not tell.  But they and their spirit voices faded together; and the night became as it had been when he walked home.

Scrooge closed the window, and examined the door by which the Ghost had entered.  It was double-locked, as he had locked it with his own hands, and the bolts were undisturbed.  He tried to say "Humbug!" but stopped at the first syllable.  And being, from the emotion he had undergone, or the fatigues of the day, or his glimpse of the Invisible World, or the dull conversation of the Ghost, or the lateness of the hour, much in need of repose; went straight to bed, without undressing, and fell asleep upon the instant.