Charles Spurgeon Commentary Mark 5:13

"And he gave them leave. And the unclean spirits came out, and entered into the swine: and the herd rushed down the steep into the sea, [in number] about two thousand; and they were drowned in the sea." — Mark 5:13 (ASV)

And forthwith Jesus gave them leave.

The devil cannot enter even a pig without Christ's leave. So he cannot tempt you, my friend, without our Lord's permission. You may rest assured that even this great monster of evil is under Christ's control. He cannot harm you until Jesus gives him leave.

There is a chain around the roaring lion, and he can only go just as far as the Lord allows him.

And forthwith Jesus gave them leave. And the unclean spirits went out, and entered into the swine: and the herd ran violently down a steep place into the sea, (they were about two thousand; )

It was strange that there should be so many swine in the country where lived God's people Israel, and since they had no right to be there and were there contrary to Jewish law, it was fitting that they should be destroyed.