Demons are never described in the Bible as trying to tempt people or corrupt them; demons in the sense of demon possessed people often express faith in Christ. This is in sharp contrast to the assumption commonly made that demons are fallen angels intent on tempting people to sin- in Pentecostal churches we hear of a shopping demon, a smoking demon, a speeding demon, etc. But this simply isn't how 'demons' are referred to in the New Testament. The Bible speaks of demons as being the idols which had been built to represent them; and it is stressed that these idols and the demons supposedly behind them don't exist. And therefore "be not afraid of them; for they cannot do evil", nor have they any capacity to in fact do anything (Jer. 10:3-6; Ps. 115:2-9).
Bullinger has some interesting comments upon the woman with an unclean “spirit of infirmity” (Lk. 13:11) that resulted in her being unable to lift herself up straight. “The negative is me, not ou; and is therefore subjective. She felt as if she could not do so…it appears, therefore, to have been a nervous disorder; and had to do with her pneuma” or mind (1). And yet she is described as having been 'bound by satan’. The ‘satan’ or adversary to her standing upright was her own mindset. And it was this spirit or mindset “of infirmity” from which the Lord released her. Here we clearly see the connection between ‘spirits’ and mental disorder or dysfunction; for ‘spirit’ in Scripture so often refers to the psychological mindset of a person.
For what it's worth, psychologists have suggested that belief in demons is rooted within the human desire to externalize our internal problems, to unload all our inner fears and anger onto some mythical creatures of our creation. I am no great fan of Freud, but some of his conclusions are at least worth referencing. He denied the literal existence of demons, but addressed the question of why people believe in them. He claimed that the belief derived "from suppressed hostile and cruel impulses. The greater part of superstition signifies fear of impending evil, and he who has frequently wished evil to others, but because of a good bringing-up, has repressed the same into the unconscious, will be particularly apt to expect punishment for such unconscious evil in the form of a misfortune threatening him from without" (2). Further he wrote: "[it is] quite possible that the whole conception of demons was derived from the extremely important relation to the dead... nothing testifies so much to the influence of mourning on the origin of belief in demons as the fact that demons were always taken to be the spirits of persons not long dead" (3). The anger, guilt and fear which is part of the mourning process therefore came to be unloaded onto the 'demons' which were imagined. Gerardus van der Leeuw, a theologian, took the idea further: "Horror and shuddering, sudden fright and the frantic insanity of dread, all receive their form in the demon; this represents the absolute horribleness of the world, the incalculable force which weaves its web around us and threatens to seize us. Hence all the vagueness and ambiguity of the demon's nature.... The demons' behaviour is arbitrary, purposeless, even clumsy and ridiculous, but despite this it is no less terrifying" (4). I am unsure whether I can agree with everything these writers suggest in this context- but it seems to me a likely enough psychological explanation for the common belief in demons. Our anger, our fear, our trembling, our fear of the unknown, of ourselves even, was somehow transformed by people into a belief that all these things existed in a tangible concrete form as 'demons' external to us. We as it were unload our own internal demons onto external, literal demons... as always, to make ourselves appear the less culpable, the less fearful and the less sinful.
Notes
(1) E.W. Bullinger, Word Studies On The Holy Spirit (Grand Rapids: Kregel, 1985 ed.) p. 63 [formerly published as The Giver And His Gifts].
(2) Sigmund Freud, "Psychopathology of Everyday Life," in The Basic Writings Of Sigmund Freud, ed. A. A. Brill (New York: The Modern Library, 1938), p. 165.
(3) Sigmund Freud, "Totem and Taboo," in The Basic Writings Of Sigmund Freud, op. cit., pp. 857-858.
(4) G. van der Leeuw, Religion In Essence And Manifestation (Princeton: Princeton University Press, 1986), pp. 134-135.