The biblical worldview is supernatural and supranatural. This means that there exist a reality that our five physical senses cannot detect. It includes the belief in the existence of both good and evil personalities without bodies, viz., God, angels and demons. The Western Christian worldview has been very much undermined by secular humanism which denies the supernatural and the supranatural. Unfortunately, such denial is due more to ignorance than to science. If science is evidence-based inquiry, then there are many instances of bizarre behaviour we observe around us that cannot be explained only in terms of psychological and medical conditions.
Like the story in the passage quoted above, the demoniac who lived among the tombs was obviously demonized. The evidence is this: the man under the control of the demons knew Jesus' divine self-identity: "What have you to do with me, Jesus, Son of the Most High God? I beg you, do not torment me" (v.28). And when Jesus asked him for his name, the demons speaking through that man, replied, "Legion". The Gospel of Mark gives us more information regarding the name Legion: "My name is Legion, for we are many." (Mark 5:9). In fact, a Roman legion is about 6,000 men. When Jesus gave the demons permission to enter the pigs, we are told there in Mark 5:13 that there were 2,000 pigs: "So he gave them permission. And the unclean spirits came out, and entered the pigs, and the herd, numbering about two thousand, rushed down the steep bank into the sea and were drowned in the sea." So, there were at least 2,000 demons in this one man alone.
Now, of course, this might seem incredulous to those who believe there is nothing beyond the natural realm. But even science does not preclude physical entities that occupy no space. One can think of the thousands of mobile microwaves and radio waves filling our atmosphere right now that occupy no physical space. But no one would deny the existence of these myriad electromagnetic waves passing through us right this very instant, waiting to come alive by entering our mobile phones or radio receivers to speak through these devices. So if we can believe in electromagnetic radiation that occupies no space, it's not too far fetched nor unscientific to believe in 6,000 spiritual entities (demons) occupying the space of one man.
Another important fact from this story is that demons entering into a human will distort the personality of that man or woman. When demons take control of a man, he is capable of doing weird things. Again for more information about the demoniac, we go to the Gospel of Mark: "He lived among the tombs. And no one could bind him anymore, not even with a chain, for he had often been bound with shackles and chains, but he wrenched the chains apart, and he broke the shackles in pieces. No one had the strength to subdue him. Night and day among the tombs and on the mountains he was always crying out and bruising himself with stones" (Mark 5:3-5). So, the demons controlling this man gave him supernatural strength, caused him to cry out loudly and cut himself with stones.
In my own encounter with demonized individuals, I have witnessed such bizarre behavior caused by demons. I remember an elderly man (perhaps in his 60s) who attended an Alpha weekend. He was a Taoist/Buddhist. He was quite jovial and we were having a very normal conversation over lunch. I asked how he found the Alpha Course thus far and he said he liked it. He then told me he was not a believer and had visited many temples in Thailand to seek for the truth. That evening, we had our Holy Spirit prayer ministry time. After the talk on "How can I be filled with the Holy Spirit?" I instructed the worship team to sing a few songs before we began to prayer ministry for the guests. The worship team didn't get very far into the singing before the elderly man who was seated at the back started to yell with a loud voice and the demons threw him onto the floor. When we got near to him, we saw that his eyes were rolling out of control. At the same time, he was thrashing all over the floor and we needed a few strong men to hold him down. I commanded the demons to stop manifesting, got the man to leave the room and we counseled him outside the room. We asked him to renounce all his past dealings with temple worship.
This is just one of many instances of people who didn't know they were demonized until they came into the presence of Jesus through the Holy Spirit. It's just like the demoniac in our story where the demons in him were crying out to Jesus not to torment them: "What have you to do with me, Jesus, Son of the Most High God? I adjure you by God, do not torment me" (Mark 5:7-8). When the Holy Spirit is present, demons are tormented by the very presence of God. Darkness and light cannot coexist--and so the demons screamed to get out of Holy Spirit presence.
But the best part of this story in Luke 8 is that after the demons were driven out, the man regained his sanity. While we are not implying that all instances of insanity are caused by demons, clearly demons can distort a person's perception of reality and cause him to suffer from delusions. But when the demons are cast out, the person will regain control over his mental faculties: "the man from whom the demons had gone [was] sitting at the feet of Jesus, clothed and in his right mind" (v.35).
Father, thank You that You have given Your church authority to cast out demons through Jesus' name so that those who have been tormented by demons may be set free. Amen.