In the Gospels generally, in (James 2:19) and in Reve 16:14 The demons are spoken of as spiritual beings, at enmity with God, and having power to afflict man not only with disease, but, as is marked by the frequent epithet "un-clean," with spiritual pollution also. They "believe" the power of God "and tremble," (James 2:19) they recognized the Lord as the Son of God, (Matthew 8:29; Luke 4:41) and acknowledged the power of his name, used in exorcism. In the place of the name of Jehovah, by his appointed messengers, (Acts 19:15) and looked forward in terror to the judgment to come. (Matthew 8:29) The description is precisely that of a nature akin to the angelic in knowledge and powers, but with the emphatic addition of the idea of positive and active wickedness.
Main reference: Smith's Bible Dictionary (1860s)
Bible Gallery and Bible Illustrations (thousands of images)