The New Testament, including Jesus Himself, clearly teaches the existence of a spiritual realm of demonic and angelic personal beings. The former are evil (sinning and rebelling from their created state as angels) and the latter are obedient servants of God.
Some people today dismiss such beings as a pre-scientific way of describing psychological dysfunctions such as hysteria or epilepsy. While it is true that at certain times in history some sects of Christianity have abused people with this doctrine of demons, it is primarily because of the actions and teachings of Jesus about nonphysical beings that we are capable of avoiding such abuse and having a more complete understanding of the universe.
The New Testament refers to angels 176 times, to Satan (or the devil) 65 times, and to demons or unclean spirits 75 times. See Matthew 1:20; 2:13; 4:1-11; 8:28-34; 9:32-35; and John 8:44 for examples of these. In view of the New Testament’s teaching, to deny the possibility of the existence of angels and demons would, by extension, lead to the rejection of all spiritual beings, including God.
Some forms of physical disease and dysfunction are attributed to demons in the Gospels, including incidents of paralysis (Luke 13:10-13); violence and bodily mutilation (Mark 5:1-20); and epilepsy (Matthew 17:14-18). It should be noted that not all diseases are associated with demons, because the New Testament differentiates between healing diseases and casting out demons. Therefore, we need to take a balanced perspective, neither denying that personal evil spirits exist nor seeing them “behind every bush.” If they are encountered, the believer in Christ will find sufficient power in Jesus to expel them.
There is a logical explanation as to why the devil and demonic beings seemed to be particularly active at the time of Jesus. He came with the power of the Kingdom of God to set individuals free from sin and remove them from the devil’s kingdom of darkness. We would expect such spiritual salvation to be met with opposition.
“Thus the demons are part of the cosmic or spiritual conflict going on behind the outward actions [by Jesus] of preaching, teaching and healing. Demons fit into the New Testament picture of what the reign of God means and the fact that salvation is not simply deliverance from physical sickness or political oppression or poverty, but at root is a deliverance from final judgment, from spiritual sin and from the oppression by evil spiritual forces connected to these things” (Walter Kaiser, Jr., et. al., Hard Sayings of the Bible [InterVarsity, 1997]).