Ok that's fine.
So, you are saying that there is absolutely no confusion during this ancient time period with modern day recognized mental illness or other behavioral disorders, that demon possession is strictly another issue, or as you have said, spiritual.
So maybe I am wrong.
What ancient people may or may not have thought about mental illness is not really relevant to talking about what the Scripture says about the nature of demonic possession, because we can establish beyond any doubt from Scripture that demonic possession symptoms could not have been just a natural condition.
Furthermore, we also see elsewhere Jesus refer to demons as entities with personalities/wills that can enter and leave individuals.
Matthew 12:
But when the Pharisees heard this, they said, “It is only by Beelzebul, the prince of demons, that this fellow drives out demons.”
25 Jesus knew their thoughts and said to them, “Every kingdom divided against itself will be ruined, and every city or household divided against itself will not stand. 26 If Satan drives out Satan, he is divided against himself. How then can his kingdom stand? 27 And if I drive out demons by Beelzebul, by whom do your people drive them out? So then, they will be your judges. 28 But if it is by the Spirit of God that I drive out demons, then the kingdom of God has come upon you.
29 “Or again, how can anyone enter a strong man’s house and carry off his possessions unless he first ties up the strong man? Then he can plunder his house.
...
“When an impure spirit comes out of a person, it goes through arid places seeking rest and does not find it. 44 Then it says, ‘I will return to the house I left.’ When it arrives, it finds the house unoccupied, swept clean and put in order. 45 Then it goes and takes with it seven other spirits more wicked than itself, and they go in and live there. And the final condition of that person is worse than the first. That is how it will be with this wicked generation.”
Matthew 8:
28 When they arrived on the other side of the lake, in the region of the Gadarenes,[v] two demonized men confronted Jesus. They lived among the tombs of a cemetery and were considered so extremely violent that no one felt safe passing through that area. 29 The demons screamed at Jesus, shouting, “Son of God, what do you want with us?[w] Leave us alone! Have you come to torment us before the appointed time?”
30 There was a large herd of pigs feeding nearby, 31 so the demons pleaded, “If you cast us out, send us into that herd of pigs.” 32 Jesus commanded, “Then go!” And at once the demons came out of the men and went into the pigs.[x] Then the entire herd of crazed pigs stampeded down the steep slope and fell into the water and drowned.
Luke 4:
33 In the synagogue there was a man possessed by a demon, an impure spirit. He cried out at the top of his voice, 34 “Go away! What do you want with us, Jesus of Nazareth? Have you come to destroy us? I know who you are—the Holy One of God!”
35 “Be quiet!” Jesus said sternly. “Come out of him!” Then the demon threw the man down before them all and came out without injuring him.
36 All the people were amazed and said to each other, “What words these are! With authority and power he gives orders to impure spirits and they come out!” 37 And the news about him spread throughout the surrounding area.
...
40 At sunset, the people brought to Jesus all who had various kinds of sickness, and laying his hands on each one, he healed them. 41 Moreover, demons came out of many people, shouting, “You are the Son of God!” But he rebuked them and would not allow them to speak, because they knew he was the Messiah.
Luke 8:
27 And when He came out onto the land, He was met by a man from the city who was possessed with demons; and who had not put on any clothing for a long time, and was not living in a house, but in the tombs. 28 Seeing Jesus, he cried out and fell before Him, and said in a loud voice, “[g]What business do we have with each other, Jesus, Son of the Most High God? I beg You, do not torment me.” 29 For He had commanded the unclean spirit to come out of the man. For it had seized him many times; and he was bound with chains and shackles and kept under guard, and yet he would break his bonds and be driven by the demon into the desert. 30 And Jesus asked him, “What is your name?” And he said, “Legion”; for many demons had entered him. 31 They were imploring Him not to command them to go away into the abyss.
32 Now there was a herd of many swine feeding there on the mountain; and the demons implored Him to permit them to enter [h]the swine. And He gave them permission. 33 And the demons came out of the man and entered the swine; and the herd rushed down the steep bank into the lake and was drowned.
34 When the herdsmen saw what had happened, they ran away and reported it in the city and out in the country. 35 The people went out to see what had happened; and they came to Jesus, and found the man from whom the demons had gone out, sitting down at the feet of Jesus, clothed and in his right mind; and they became frightened. 36 Those who had seen it reported to them how the man who was demon-possessed had been
made well. 37 And all the people of the country of the Gerasenes and the surrounding district asked Him to leave them, for they were gripped with great fear; and He got into a boat and returned. 38 But the man from whom the demons had gone out was begging Him that he might [j]accompany Him; but He sent him away, saying, 39 “Return to your house and describe what great things God has done for you.” So he went away, proclaiming throughout the whole city what great things Jesus had done for him.
We also see this with Paul:
Acts 16:
16 Once when we were going to the place of prayer, we were met by a female slave who had a spirit by which she predicted the future. She earned a great deal of money for her owners by fortune-telling. 17 She followed Paul and the rest of us, shouting, “These men are servants of the Most High God, who are telling you the way to be saved.” 18 She kept this up for many days. Finally Paul became so annoyed that he turned around and said to the spirit, “In the name of Jesus Christ I command you to come out of her!” At that moment the spirit left her.
19 When her owners realized that their hope of making money was gone, they seized Paul and Silas and dragged them into the marketplace to face the authorities. 20 They brought them before the magistrates and said, “These men are Jews, and are throwing our city into an uproar 21 by advocating customs unlawful for us Romans to accept or practice.”
The fact is, Scripturally, we see that demons can cause both physical sickness and mental illness. But we see also from Scripture that demons are spiritual entities with personalities and that they result in other effects on individuals beyond just illness (like unnatural strength or knowing about things they don't know in the natural).
So, "demon" is not just some word or concept that people just started attaching to any kind of illness out of ignorance for what was medically going on. In many cases a demon is actually behind things we would today recognize as medical illnesses and dealing with those spirits will actually bring physical/mental healing. We know this from modern experience where ministers of God do exactly that and people are supernaturally healed in ways that modern medicine can't explain. And we know Scripturally that is what we see Jesus and the apostles doing too.
