Question: "Why did Jesus allow the demons to enter the herd of pigs?"
Answer: The story of Jesus casting the legion of demons into a herd of pigs is found in Matthew 8:30-37; Mark 5:1-20; and Luke 8:27-38. Only Matthew mentions the more prominent of the two demoniacs involved. Demoniacs were persons whose minds came under the control of an evil spirit or spirits. That such phenomena were especially prominent during the days of Christ’s earthly ministry is consistent with Satan’s efforts to counteract God’s program. It also allows us to witness the spiritual warfare in which our Savior was constantly engaged. Demons knew exactly who Jesus was—"Son of God"—and were aware of their ultimate doom (Matthew 8:28-29).
As Jesus was traveling in the hilly region east of the Jordan River, the path of this man who was controlled by demons and lived among the tombs crossed that of Jesus. Because of the physical strength the demons gave the man, he was able to break and throw off the chains with which people tried to bind him. When the demons begged Jesus to let them go into a herd of pigs, He gave them permission. They entered the pigs, rushed down the steep bank into the lake, and were drowned. Jesus thereby made known His authority and thwarted whatever evil purpose the demons had.
Why the demons begged to be allowed to enter the swine is unclear from the account. It could be because they didn’t want to leave the area where they had been successful in doing their mischief among the people. Perhaps they were drawn to the unclean animals because of their own filthiness. The demons may have made this strange request because it was their last chance to avoid confinement in the Abyss, the place of confinement to which evil spirits are doomed (Revelation 9:1-6). Whatever their reasoning, it is clear from the account that demons had little power of their own and were unable to do anything without Jesus’ permission. As Christians, we can take comfort in the knowledge that the forces of the enemy of our souls are under the complete control of God and can only act in ways He allows.
The Bible doesn’t explain to us Jesus’ reasoning, but displaying His sovereign power over demons could be one reason why Jesus sent them into the pigs. If the pigs’ owners were Jews, Jesus could have been rebuking them for violating Mosaic law which forbids Jews from eating or keeping unclean animals such as swine (Leviticus 11:7). If the swineherds were Gentiles, perhaps Jesus was using this miraculous event to show them the malice of evil spirits under whose influence they lived, as well as displaying His own power and authority over creation. In any case, the owners were so terrified to be in the presence of such spiritual power that they made no demand for restitution for the loss of their property and begged Jesus to leave the region. The people were awe-struck but unrepentant—they wanted no more of Jesus Christ. This shows the hardness of their hearts and their desire to remain in sin. The healed demoniac, on the other hand, demonstrated the true faith and repentance of a changed heart and begged to be allowed to follow Jesus. Perhaps the unmistakable difference between the saved and the unsaved was an object lesson for the disciples and all who witnessed the event. Jesus sent the healed man away, giving him a commission that he joyfully obeyed: "Go home to your family and tell them how much the Lord has done for you, and how he has had mercy on you" (Mark 5:17-20).