Many in the modern world consider the existence of demons to be lunacy, a relic from the dark ages, a superstition.
On the other hand, many years ago I took the three men with developmental disability who I had charge of to a Christian coffeehouse in town one Friday evening. It was during the 1980’s, at the height of the charismatic renewal, and a Protestant ministry. I too was Protestant back then. Not many people were there, and the two men who were wanted to pray for the three men with me for their deliverance. I tried to explain to them that their disability was from physical rather than demonic sources, but they did not seem convinced.
John Sanidopoulos’ article briefly explains the difference between demonic possession and mental illness from an Orthodox Christian perspective.
Here are other Orthodox Christian presentations on this topic:
A Discussion: On Mental Illness. OrthodoxChristianity.net
Concerning Evil Spirits and “Internal Demons” The Orthodox Christian Teaching about the Nature and Activity of Satan and his Demons. Our Responsibility for Our Choices and Actions, and How we Struggle Against “Self-temptation” and Addiction. by Archbishop Lazar Puhalo. Synaxis Press