When the unclean spirit is gone out of a man, he walketh through dry places, seeking rest, and findeth none. Then he saith, I will return into my house from whence I came out; and when he is come, he findeth [it] empty, swept, and garnished. Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. Even so shall it be also unto this wicked generation.

Matthew 12:43-45

When the unclean spirit is gone out of a man, he walketh through dry places, seeking rest; and finding none, he saith, I will return unto my house whence I came out. And when he cometh, he findeth it swept and garnished. Then goeth he, and taketh to him seven other spirits more wicked than himself; and they enter in, and dwell there: and the last [state] of that man is worse than the first.

Luke 11:24-26

Nature abhors a vacuum. A life left empty and unoccupied will be filled with something. Jesus uses the occasion of casting out a devil from a man to teach a lesson in repentance and conversion.  

In this parable Jesus tells about an unclean spirit that has been cast out of a man. The spirit then wanders about looking for another place he is comfortable. This means he is looking for another man to possess! Unsatisfied, the spirit returns to the man he formerly possessed. He finds that the man hasn’t occupied his life with other things, and he is easily filled again. This time, the demon invites his friends over, and the life of the man is far worse now than before.

The main point of Jesus’ teaching is that repentance does the child of God no good if he does not occupy his life with a new behavior. It is not enough to simply repent and declare that we will no longer commit a particular sin. If we do not find a good behavior to replace a bad one, we will return to the sin.

The Pharisees had a “form of religion” that had no true moral foundation. They saw no need of repentance but would turn away from sin to appear righteous, only to come back later and add to their indulgence.  

Children of God are all in the same danger of being influenced again by spirits and sins that we once turned away from. We need to replace that old behavior with new godly behaviors. Consider Paul’s instruction to the Ephesians to replace ungodly behavior with beneficial love toward others:

Wherefore putting away lying, speak every man truth with his neighbour: for we are members one of another. Be ye angry, and sin not: let not the sun go down upon your wrath: Neither give place to the devil. Let him that stole steal no more: but rather let him labour, working with his hands the thing which is good, that he may have to give to him that needeth. Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying, that it may minister grace unto the hearers. And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption. Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all malice: And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ’s sake hath forgiven you.

Ephesians 4:25-32