And Jesus asked him, “What is your name?” He replied, “My name is Legion, for we are many.”
Mark 5:9 My name is Legion. The devil was compelled by Christ to pronounce this word, that he might more fully display the greatness and excellence of his grace. There must have been good reasons why this man should have endured so severe a punishment as to have an army of devils, so to speak, dwelling within him.
We have here an instance of Christ's dispossessing the strong man armed, and disposing of him as he pleased, to make it appear that he was stronger than he. This he did when he was come to the other side, whither he went through a storm; his business there was to rescue this poor creature out of the hands of Satan, and when he had done that, he returned.
Commenting on Mark 5:1-20
And he asked him, what is thy name?.... Which question Christ put, not for his own sake; for he was not ignorant of his name, nor of the number of the unclean spirits which were in the man; but partly, that it might be known what a miserable condition this poor man was in, being infested, and vexed with such a large company of devils...