"You are worried and troubled about many things." -Luke 10:41-42
Here is what St. Irenaeus of Lyons of the second century says of the work of the devil, "He had indeed been already accustomed to lie against God, for the purpose of leading men astray." (Against Heresies, 5.23.1) "To lead men astray," he says!
Does your salvation concern you? Since this is what is truly important! (not the size of a demon but its intent).
Scripture very clearly teaches that demons are real (In the Gospels alone, the word "demon" is used thirty-two times, "devil" and "Satan" both appear fourteen times, and the phrase "the evil one" appears five times.).*
The Apostle Peter calls to each Christian: "Be sober, be vigilant; because your adversary the devil walketh about as a roaring lion, seeking whom he may devour" (1 Peter 5:8).
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* New Testament references to demons:
The freeing of people from the devil was the first work that our Lord Jesus Christ took on (Matt. 4:24, 8:16, 9:32-34; Mark 1:32-34; Luke 4:41, 8:2, 11:4) Specifically, we have the following accounts: the healing of the possessed youth (Matt. 17:14-21), the daughter of the Canaanite woman (Mark 7:24-29), and the two possessed Gadarenes (Matt. 8:28-34, Mark 5:1-19). In the last incident it is worth noting that not one but a whole legion of demons (i.e., a great number of them) became established in a man. Jesus prepared his disciples to drive out demons. He sent them in pairs into the cities and villages with the power to drive out the unclean spirits (Matt. 10:1, Mark 6:7, Luke 9:1). When they returned they reported. "Lord, even the devils are subject unto us through Thy name" (Luke 10:17). The Lord foretold before His Ascension into Heaven that the breaking of diabolic power would be a clear sign of the Church: "And these signs shall follow them that believe: In my name shall they cast out devils ..." (Mark 16:17). In the book of Acts we find many examples of the holy Apostles dealing with demons, including the healing of the possessed by the Apostle Peter (Acts 5:16), by the Apostle Philip when "unclean spirits, crying with a loud voice, came out of many who were possessed with them" (Acts 8:7), and later by the Apostle Paul, who expelled a spirit of divination from a young woman (Acts 16:16-18). Moreover, the blessing of God acted so abundantly in those times that when, for example, personal things that belong to the Apostle Paul were laid on the possessed, the demons went out of them (Acts 19:12).