In our modern and "enlightened" age, we tend not to think in terms of spirits. Anything which has to do with aberrant behavior must be some form of psychosis or chemical unbalance that we can treat with counseling or medication.
But if the Bible is true, it is all true. That includes this story about the Gerasene. The man was not psychotic or chemically unbalanced. He was demon-possessed.
There are such things as demons. There is spiritual warfare. We need to be aware that we have a spiritual enemy who seeks our eternal destruction.
If we believe in God, we must believe that the devil exists. We see him in conversation with Eve in Genesis 3. We are told of his fall in Isaiah 14. There are multiple references to Satan in the New Testament. And the entire Book of Job is a chronicle of spiritual warfare.
It is part of Satan's first lie in the Garden of Eden, that mankind can know all things, find answers comprehensible to the finite mind for all questions, discover cures to all diseases and maladies. When we buy into this arrogance of the intellect, we fall directly into the enemy's hands.
Not everything is part of our physical, tangible world. There is a spiritual plane of which we know far too little. But all we need to know of it is contained in the Bible. We'll not find it in psychology texts. Nor will we find beneficial information in new age books of spirituality.
Our hope and our confidence comes from the fact that God is Lord of the spiritual realm, just as He is Creator and Lord of this world. When we serve Him, when we rely solely on Him, He will preserve us from the wiles of the enemy.
Stop our ears from hearing the enticements of the enemy. Shield our eyes from the lust to which he would tempt us - whatever form it may take. Fill our eyes and ears and hearts with Your Holy Spirit so that there is no room for anything else.
In Jesus' most precious name.