Matthew 12:43-45
http://www.usccb.org/nab/bible/matthew/matthew12.htm
43
"When an unclean spirit goes out of a person it roams through arid regions searching for rest but finds none.
44
Then it says, 'I will return to my home from which I came.' But upon returning, it finds it empty, swept clean, and put in order.
45
Then it goes and brings back with itself seven other spirits more evil than itself, and they move in and dwell there; and the last condition of that person is worse than the first. Thus it will be with this evil generation."
Great reference, especially considering the unbeliever who has been possessed and then relaxed and then re-entered.
One great thing about Christ, though, is that until a human suffers the first death, separation of soul from body, that human still has an opportunity for salvation, then after salvation, no matter how heinous the sin, one still has an opportunity to confess that sin, turn away from sin, and return to a fellowship with God. Nothing is more powerful than the love of Him.
(Those who continue to rebel after salvation might risk sinning to the point that within His plan, they serve no more useful purpose, or if you will, commit a sin unto death and are brought home to the Lord, leaving many rewards on the table as eternal memorials to the foolishness of those who stray from Him and His plan.