"When the unclean spirit has gone out of a man, he passes through waterless places seeking rest; and finding none he says, 'I will return to my house from which I came.' And when he comes he finds it swept and put in order. Then he goes and brings seven other spirits more evil than himself, and they enter and dwell there; and the last state of that man becomes worse than the first."
Luke 11:24–26 (RSV)
We rarely speak of demons, even less of possession, though I am confident that both are realities. The lesson to learn from the words of Jesus still very much applies to us today.
Sin inhabits our lives and possesses us as would a demon. We cannot rid ourselves of sin or its effects on our lives. It is the condition which we cannot escape on our own. Sin must be driven out by a power greater than ourselves. It takes an act of mercy and grace to relieve us of its hold on us.
Jesus brings forgiveness to us through His death and resurrection. It is by His victory over sin that we are set free from its corrosive effects. We know that sin will return if given a chance.
Like a demon, sin is always looking for an opportunity to return. Even though we are free of sin for a time, if our souls are not filled once it has been cast out, sin will return, often with a significant impact on our lives.
The Holy Spirit has come to create faith so we might trust the promises of God. The Spirit would reside in us, filling the place where sin once held sway. The Spirit works sanctification in our lives so we might become more and more like our Lord.
Should we ignore the Spirit's work within us, then sin, like a demon, will return, seeking to repossess its old place within us. We are forgiven so that we might become more and more like Jesus. The Father's desire for our sanctification is at work as the Holy Spirit indwells within us. As the Spirit is working in our being, there will be less and less opportunity for sin to take hold.