What is the unforgivable sin?
Interpretations 13 to 24, all different.

"The Pharisees mentioned by Matthew militantly hated Christ and attributed His miracles to Beelzebub, the prince of demons. ... with premeditation and persistence, they willfully blasphemed the Holy Spirit’s testimony that Christ was the Son of the living God. ... "

"Passages like Matthew 12:22-37, Mark 3:20-30, and Luke 12:10 all have one thing in common. They consistently portray people who knowingly persist in rejecting Christ’s messianic mission despite His miracles and the ministering power of the Holy Spirit. ... Therefore, their rejection of Christ epitomized the blasphemy of the Holy Spirit." ₁

"Verse [Mark3:]30 is crystal clear: those whom Jesus warned about the danger of committing this sin genuinely believed that Jesus had a demon. Imagine knowing the Scriptures inside out and yet being so spiritually blind as to be convinced that Jesus’ miracles were evil and that the Savior of the world – the One on whom their eternal destiny hinges – is demon possessed!" ₂

"Jesus said 'don't go looking' where the people are pointing because the Kingdom is within you. He insisted that you cannot worship in houses built by human hands or by the works of human hands. He said that you MUST worship the Spirit God in the place of your human spirit as it is devoted to the Spirit OF truth." ₃

"Is Replacing The Word of Christ with "Music" The Unforgivable Sin? It is in the Oldest Traditions That Instruments Came From the Devil. The book of Enoch along with the book of Jubilees and many ancient views of the Jews is that God will come with ten thousand of His saints and the explicit sin is refusing to obey the Word of God and replacing it with mixed-sex choirs and instrumental music to seduce the holy people into the valley of the Cainites. ... This has the effect, as in many Old Testament examples, of telling Christ: 'We do not want to hear from you.' This is a claim that He and His words are destructive for our religious systems. And this is the unforgivable sin." ₄

"I am also equating the unforgivable sin to the sin of the rapidly growing New Age movement today who's tentacles are even now reaching into the Christian denominations and drawing out the spiritually baser fellows among them, wherein man presumes himself to become as God." ₅

"I believe, the only unforgivable sin is the rejection of Jesus Christ's offer of salvation, His free gift of eternal life, and thus, His forgiveness from sin. If you don't accept His gift, you cannot be forgiven." ₆

"Even their best ‘literal’ caricature of Christ Speaking to the storm is to go out in a hurricane and yell: Stop it!, neither believing in the real existence of what they speak toward nor in the power of the Word of God –and then laughing at God because of their own failures as if their powerless, non-creating speech of sin were the Same as Himself as Word of God–and then saying their ‘literal’ is not an alternate literal of spelling and pronunciation of God as being anti-Christ/non-creating speech of sin."

"That is the only unforgivable sin." ₇

"A broader interpretation argues that the sin doesn’t lie in attributing Jesus’ power to demons but the inability to properly distinguish good from evil or God from Satan." ₈

"... the gifts of the Spirit are from the Holy Spirit. There are some people who come on very negative about some of the gifts of the spirit, especially speaking in tongues. ... It is my opinion that if you go around saying a bunch of negative things about tongues, or of the other gifts, you are getting close to blaspheming the Holy Spirit. So, my advice is DO NOT BADMOUTH THE GIFTS OF THE SPIRIT." ₉

"No": 33 responses. Two typical comments were:

"It's not a 'sin' at all -- it's human nature. How can being what God made you (if there is a God and if he made us) a sin? "No, while of course it is a sin, it is not unforgivable."

"Yes": 1 response.

"Yes. God meant for men to be with women. its in the Bible, no matter how much you want to deny it. its there.

"Undecipherable": 2 responses. ₁₁

The question was asked on the Lesbian, Gay, Bisexual and transgender section of the Society & Culture section of Yahoo! Answers. So, the responders might have been biased.