The Lord taught that “blasphemy against the Holy Ghost” is a sin that can’t be forgiven:
30 He that is not with me is against me; and he that gathereth not with me scattereth abroad.
31 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. (Matthew 12)
The unpardonable sin is called “blasphemy against the Holy Ghost” because it is the act of turning away from our understanding of what is right.
The Lord forgives your past sins on the condition that you forsake them and remain reconciled to him as he teaches you more about himself, the Father, and all of creation. If you violate this condition by knowingly deviating from what you believe he would do in your situation, your previous sins once again become unforgiven.
Whenever you knowingly do anything less than the best you can imagine, you commit the unpardonable sin.