See this account of the demoniacs fully explained in the notes at Matthew 8:28-34.
He had been often bound with fetters and chains - Efforts had been made to confine him, but his great strength - his strength increased by his malady - had prevented it. There often appears to be a great increase of strength produced by insanity, and what is here stated in regard to this maniac often occurs in Palestine and elsewhere now. Dr. Thomson (“The Land and the Book,” vol. i. p. 213) says respecting this case: “There are some very similar at the present day - furious and dangerous maniacs, who wander about the mountains, and sleep in tombs and caves. In their worst paroxysms they are quite unmanageable and prodigiously strong.” Luke 8:27 says of him that “he were no clothes,” or that he was naked, which is also implied in the account in Mark, who tells us that after he was healed he was found “clothed and in his right mind,” Mark 4:15. This is often a striking characteristic of insanity. Dr. Pritchard (on “Insanity,” p. 26) quotes from an Italian physician’s description of raving madness or mania: “A striking and characteristic circumstance is the propensity to go quite naked. The patient tears his clothes to tatters.” So Dr. Thomson (“The Land and the Book,” vol. i. p. 213) says: “It is one of the most common traits in this madness that the victims refuse to wear clothes. I have often seen them absolutely naked in the crowded streets of Beirut and Sidon. There are also cases in which they run wildly about the country and frighten the whole neighborhood. These poor wretches are held in the greatest reverence by Muslims, who, through some monstrous perversion of ideas, believe them to be inspired and peculiarly holy.”
Cutting himself with stones - These are all marks of a madman - a man bereft of reason, a wretched outcast, strong and dangerous. The inspired penman says that this madness was caused by an unclean spirit, or by his being under the influence of a devil. That this account is not irrational, see the notes at Matthew 4:24.
Worshipped him - Bowed down before him; rendered him homage. This was an acknowledgment of his power, and of his control over fallen spirits.
My name is Legion - See the notes at Matthew 8:29.
Sitting, and clothed, and in his right mind - There could be no doubt of the reality of this miracle. The man had been well known. He had long dwelt among the tombs, an object of terror and alarm. To see him all at once peaceful, calm, and rational, was proof that it was the power of God only that had done it.
They were afraid - They were awed, as in the presence of God. The word does not mean here that they feared that any evil would happen to them, but that they were affected with awe; they felt that God was there; they were struck with astonishment at what Jesus had done.
Jesus suffered him not - Various reasons have been conjectured why Jesus did not suffer this man to go with him. It might have been that he wished to leave him among the people as a conclusive evidence of his power to work miracles. Or it might have been that the man feared that if Jesus left him the devils would return, and that Jesus told him to remain to show to him that the cure was complete, and that he had power over the devils when absent as well as when present. But the probable reason is, that he desired to restore him to his family and friends. Jesus was unwilling to delay the joy of his friends, and to prolong their anxiety by suffering him to remain away from them.
In Decapolis - See the notes at Matthew 4:25.
How great things ... - This was the natural expression of right feeling at being cured of such a calamity. So the desire of sinners freed from sin is to honor Jesus, and to invite the world to participate in the same salvation, and to join them in doing honor to the Son of God. Compare Psalms 66:16.