After we cast demons out of someone, we need to command the demons to stay out but most importantly, we must tell the delivered person to sin no more. Repentance, which is turning away from sin, is critically important. I made this mistake with one person and he got demonised again:
1 John 5:17-18 King James Version (KJV)
17 All unrighteousness is sin: and there is a sin not unto death.
18 We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not.
The person must keep themselves pure and undefiled otherwise, seven other spirits will enter in:
Matthew 12:43-45 King James Version (KJV)
43 When the unclean spirit is gone out of a man, he walketh through dry places, seeking rest, and findeth none.
44 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.
45 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.
It’s critical to keep the mind undefiled after casting demons out of someone. I learned this the hard way.