What is the unforgivable sin according to the Bible?

The unforgivable sin is articulated in Matthew 12:31-32, where Jesus states that blasphemy against the Holy Spirit will not be forgiven. This refers to a deliberate and persistent rejection of the Spirit's testimony regarding Christ and His work of salvation. Essentially, it is the willful choice to remain in unbelief despite the evidence of God's grace and truth presented through the Holy Spirit. Such a heart is hardened against the truth, making it impossible for the individual to see and respond to God's offer of mercy and salvation, thereby leading them to eternal condemnation.