In the doctrine Church of Jesus Christ of Latter-day Saints (commonly called the "Mormon" Church), the only unpardonable sin is blasphemy against the Holy Ghost. (see New Testament, Matthew 12:31 and Mark 3:29)
Blasphemy against the Holy Ghost can only be committed by those who have received all the ordinances of the Church and who have received a sure knowledge of Jesus Christ. (Book of Mormon, 2 Nephi 31:14) It involves openly and knowingly denying the Holy Ghost (Book of Mormon, Alma 39:5), to willfully sin (New Testament, Hebrews 10:26), and to lie to God (Book of Mormon, Jacob 7:19) and be in open rebellion to him (Book of Mormon, Mosiah 2:37). Joseph Smith said that to commit the unpardonable sin, one "must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against him... He has got to say that the sun does not shine while he sees it; he has got to deny Jesus Christ when the heavens have been opened unto him, and to deny the plan of salvation with his eyes open to the truth of it." It is generally believed that this sin is extremely rare and is more serious than general apostasy or unbelief.
Murder of innocent people (outside of a war setting) and adultery are considered the next horrible sins, but they are forgivable by repentance, as with every sin.
You can read the scripture references and Joseph Smith quote at the "Related Links" below.
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