“Demons” (shêd) are referred to only a few times in the Old Testament. As far as I can tell, they are always connected with paganism and idol worship. In Deuteronomy 32:17, we read, "They sacrificed to demons (shêd) that were not God, to gods they had never known, to new gods that had come recently, whom your fathers had never dreaded " (cf. 1 Cor. 10:14, 20-21). Here we find that the first time “demons” are mentioned in Scripture they are referred to as being equivalent to the false gods of the Gentile nations, to which unfaithful Israel is represented as offering sacrifices. Similarly, in Psalm 106:36-38 we read:
“They served their idols, which became a snare to them. They sacrificed their sons and their daughters to demons (shêd) they poured out innocent blood, the blood of their sons and daughters, whom they sacrificed to the idols of Canaan, and the land was polluted with blood.”
Here, the apostate people of Israel are said to have sacrificed their own children to the idols of Canaan. Whereas in Deuteronomy 32:17 “demons” are referred to as being equivalent to false gods, here they are made equivalent to “idols.” Of course, there is no inconsistency in this, since “idols” are nothing more than visible, tangible representations of pagan gods. But the gods of the heathen nations have no real existence; they are not really “gods” at all, but fictitious non-entities, existing only in the darkened imaginations of pagan man. But if (as these verses suggest) “demons” are equivalent to these false gods of the heathen (as well as the idols which represent them), and such pagan gods have no real existence, then would it not follow that “demons” also have no real existence?
While “demons” are twice referred to in the Old Testament, it is significant that “demonic possession” is not referred to at all. There is not a single instance of demonic possession to be found in all the inspired pages of the Old Testament. Some have seen possible evidence of “demonic oppression” (though not possession) in the account of King Saul and the “evil spirit from the LORD” (see 1 Sam 16:14ff; 18:10; cf. 19:9; Judges 9:23). However, there is no reason to understand this “spirit” (ruach) to have been “demonic” (as is commonly understood), or even to have been a personal being. Both in the Old and New Testament, the word “spirit” (ruach and pneuma, respectively) can signify several different things depending on the context, one of which is a mental disposition or state of mind (e.g., Deut 34:9; Num 5:14, 30; 1 Sam 1:15; 1 Kings 21:5; Psalm 51:17; Prov 16:9, 18, 19; Eccl 1:14; 7:9; Isa 11:2; 19:14; 61:3; Rom 11:8; 1 Cor 4:21; Gal 6:1; Eph 4:23; Phil 2:19; 2 Tim 1:7; 1 Pet 3:4; 1 John 4:6). Understood in this sense, the “evil spirit” from God that afflicted Saul simply referred to the troubled state of mind which God allowed Saul to be subject to because of his sin. Jamieson, Fausset, and Brown summarize this point well in their commentary: “His own gloomy reflections—the consciousness that he had not acted up to the character of an Israelitish king—the loss of his throne, and the extinction of his royal house, made him jealous, irritable, vindictive, and subject to fits of morbid melancholy.” That this is the true sense is evident from the fact that David’s soothing music is said to have refreshed Saul and made him well, and caused the “evil spirit” to depart from him (1 Sam 16:23).
Not only is there no account of anyone being possessed by a demon in the Old Testament, there is no indication given that demon-possession would ever come to be a problem among the Jewish people that would need to be addressed or dealt with. Nowhere do any of God’s prophets reveal anything about this subject. Search all we may, a “how-to guide” for performing exorcisms is simply not to be found in the Law and the Prophets. Remarkably, however, demonic possession - and the exorcising of demons from those possessed - appears a number of times in the synoptic Gospels and the book of Acts, without any explanation at all. Sometime after the OT cannon was completed, it seems demonic possession became a rather common thing among the Jewish people - so common, in fact, that performing exorcisms developed into more-or-less of an art (as it would later become among the Roman Catholics).