Unforgivable sin

The wilful, outwardly expressed and impenitent slander against the Holy Spirit, when Jesus Christ's mighty works, clearly performed by the power of the Holy Spirit, are attributed to Satan, thus subjecting Christ to public disgrace.

The unforgivable sin is slandering the Holy Spirit

Matthew 12:31–32 parallel Mark 3:28–29 See also Luke 12:10; Hebrews 10:26–29

Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. Whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the one to come.

The unforgivable sin is attributing Jesus Christ's miracles to Satan

Mark 3:30 See also Matthew 12:24 parallels Mark 3:22, Luke 11:15

Jesus made this statement because they were saying, “He has an unclean spirit.”