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Karras was using his psychiatric – plus his possession research – knowledge, which told him that it is usually psychotics who claim to be the Devil himself. He relates Regan’s claim to be THE Devil with the megalomaniac symptomology of psychotics who claim identity with great figures, e.g., Napoleon Bonaparte (or, say, one of the Pharaohs, or Jesus Christ).

Apparently, in Karras’s mind, the case for “genuine” possession increases by some degree if the patient claims to be a relatively unimportant, anonymous, obscure demon, because there is not the huge ego-investment that is involved in claiming identity with the colossally important figure of THE Devil, Satan, the Prince of This World, the Most Evil One, etc.

In the novel, however, the demon changes its claim – it claims not to be THE Devil, but just one of a “legion” of many little demons. Even in the movie, the demon says that there is more than one personality – whether or not the Devil – in Regan: the reference is to “us”, not “me” (an exorcism would bring Karras together not with Regan, but with “us”).

This doesn’t prove that the possession is genuine, but it indicates that the “invading personality” is a liar – one of the attributes of both Satan and his smaller minions…