Matthew 12

The Lord teaches concerning sin against the Holy Ghost -

"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."  (Matthew 12:43–45)

Elder Bruce R. McConkie helps us understand the above three verses as follows -

“JST Matthew 12:37–39. Having already taught that every sin shall be forgiven except the sin against the Holy Ghost, Jesus now illustrates why. In effect he says: ‘If you gain a perfect knowledge of me and my mission, it must come by revelation from the Holy Ghost; that Holy Spirit must speak to the spirit within you; and then you shall know, nothing doubting. But to receive this knowledge and revelation, you must cleanse and perfect your own soul; that is, your house must be clean, swept, and garnished. Then if you deny me by speaking against the Holy Ghost who gave you your revelation of the truth, that is if you come out in open rebellion against the perfect light you have received, the Holy Ghost will depart, leaving you to yourself. Your house will now be available for other tenancy, and so the evil spirits and influences you had once conquered will return to plague you. Having completely lost the preserving power of the Spirit, you will then be worse off than if you had never received the truth; and many in this generation shall be so condemned’” (Doctrinal New Testament Commentary, Volume 1, p. 276).