Matthew 12

Speaking to the Pharisees, the Savior taught -

"Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.
32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come."  (New Testament | Matthew 12:31 - 32)

Then adding the explanation -

"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."  (New Testament | Matthew 12:43 - 45)

The Prophet Joseph Smith translated with comments by David J. Ridges -

37 [Note: This verse is entirely missing from the Bible.] Then came some of the scribes [Jewish religious leaders who specialized in interpreting the gospel doctrines and laws] and said unto him, Master, it is written that, Every sin shall be forgiven; but ye say, Whosoever speaketh against the Holy Ghost shall not be forgiven. And they asked him, saying, How can these things be? [In other words, what you, Jesus, are teaching contradicts our traditional written doctrines.]
38 And he said unto them, When the unclean spirit is gone out of a man, he [the evil spirit] walketh through dry places, seeking rest and findeth none; but when a man speaketh against the Holy Ghost [when a man reverts back to his evil ways to the extent of denying the Holy Ghost], then he [the evil spirit] saith, I will return into my house [the man in whom the evil spirit used to reside] from whence I came out; and when he [the evil spirit] is come, he findeth him [the man in whom the evil spirit formerly resided] empty, swept and garnished [was cleansed from sin, with a sure testimony given him by the Holy Ghost, but who is now speaking against that testimony]; for the good spirit [the Holy Ghost] leaveth him unto himself.
39 Then goeth the evil spirit, and taketh with himself seven other spirits more wicked than himself; and they enter in and dwell there [with the man who has denied the Holy Ghost]; and the last end of that man is worse than the first [the man is worse off now than he was before he gained a sure testimony from the Holy Ghost]. Even so shall it be also unto this wicked generation." (JST 12:37-39)

Elder Bruce R. McConkie explained -

“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)