In this miracle, (Matthew 8:28-34; Mark 5:1-20; and Luke 8:26-39), Jesus Christ commands unclean spirits to come out of two men. Demons must obey Jesus even if people do not. Even so, the demons do not obey Christ's command immediately. They object, unwilling to abandon their victims. Christ could have compelled them to come out of the men immediately if He had wished, but the men may not have been able to survive the exorcism on their own strength.
In a separate incident, Mark 9:26-27 informs us, "Then the spirit [one demon] cried out, convulsed him greatly, and came out of him. And he became as one dead, so that many said, 'He is dead.' But Jesus took him by the hand and lifted him up, and he arose." Here, the exorcism had induced convulsions in the man as a single demon left him. Both exorcisms were under Christ's control; He used His great power but with wise, gentle, and cautious concern.
Jesus addresses the men as if they are possessed by a single spirit, but the demon's answer, giving his name as "Legion," shows that he led a company of demons. Mark records that the swine that the demons entered after their exorcism numbered about two thousand (Mark 5:13). If this was the number inhabiting the two men—each with its own personality, all under the power of one will, animated by one purpose and united in operation—then the plight of the two men must have been horrific in the extreme.
Martin G. Collins
The Miracles of Jesus Christ: Two Demon-Possessed Men Healed (Part Three)
Other Forerunner Commentary entries containing Mark 5:4: