According to Jesus’ words in Matthew 12.31, the unforgivable sin is “the blasphemy against the Spirit.” The sin He was confronting was the Pharisees’ deliberate rejection of that which they knew to be of God (see John 11.48; Acts 4.16). They could not deny the reality of what the Holy Spirit had done through Him, so they attributed to Satan a work that they knew was of God (v. 24; Mark 3.22).
Someone never exposed to Christ’s divine power and presence might reject Him in ignorance and “it will be forgiven him” (v. 32)—assuming the unbelief gives way to genuine repentance. Even a Pharisee such as Saul of Tarsus could be forgiven for speaking “against the Son of Man” or persecuting His followers—because his unbelief stemmed from ignorance (1 Tim. 1.13).
But those who know His claims are true and reject Him anyway sin “against the Holy Spirit”—because it is the Holy Spirit who testifies of Christ and makes His truth known to us (John 15.26; 16.14, 15). No forgiveness was possible for these Pharisees who witnessed His miracles firsthand, knew the truth of His claims, and still blasphemed the Holy Spirit—because they had already rejected the fullest possible revelation (Heb. 6.4–6; 10.29).