The Greek conception of a daemon (δαίμων) appears in the works of Plato and many other ancient authors, but without the evil connotations which are apparent in the Septuagint translation of the Hebrew Bible and in the Greek originals of the New Testament. The medieval and neo-medieval conception of a "demon" in Western civilization derives seamlessly from the ambient popular culture of Late (Roman) Antiquity: Greco-Roman concepts of daemons that passed into Christian culture are discussed in the entry daemon. The Hellenistic (see Hellenistic civilization) "Demon" eventually came to include many Semitic and Near Eastern gods as evaluated by Christianity.
The Greek δαιμονίζομαι daimonizomai (Mat. 8:28, Mat. 8:33, Mat. 9:32, Mar. 5:15, Luke 8:36, etc.) means, "to be under the power of a demon." The Greek word δαιμόνιον daimonion means, "evil spirits or the messengers and ministers of the devil"
According to a Jewish opinion which passed over to Christians, the demons are the gods of the gentiles and the authors of idolatry, hence:
17 They sacrificed to demons that were no gods, to gods they had never known, to new gods that had come recently, whom your fathers had never dreaded. –Deut. 32:17