According to Christian belief, blasphemy against the Holy Spirit is the sin which will not be forgiven by God. It is conscious and hardened opposition to the truth, leading one away from humility and repentance, and without repentance, there can be no forgiveness. The sin of blasphemy against the Spirit is the ongoing hardening of ones heart against the Holy Spirit who is trying to lead them to repent of sin and believe in Christ. It is not simply saying something bad about the Holy Spirit, but a persistent rejection of the convicting work of the Spirit, whose job it is to expose our sin and lead us to accept Christ.

In the context of the Gospels of Matthew and Mark, blasphemy against the Spirit is the sin of attributing to Satan what is the work of the Spirit of God, such as when the Pharisees earlier accused Jesus of driving out demons only by the power of Beelzebul, the prince of demons. The Catholic Encyclopedia cites Matthew 12:22–32; Mark 3:22–30; Luke 12:10 as the biblical references to blasphemy against the Holy Spirit.

Blasphemy against the Holy Spirit is not a wayward word uttered in a moment of anger, nor is it some other dramatic sin, like murder, or suicide, or adultery. Rather, it is ongoing hardening of ones heart against the Holy Spirit who is trying to lead them to repent of sin and believe in Christ. It is an issue of the heart that manifests in ones words and actions.

It is important to note that not all verbal expressions of skepticism, anger, disobedience, or disrespect toward the Holy Spirit can be categorized as the blasphemy of the Holy Spirit. The blasphemy of the Holy Spirit can only come from a heart that is so hard and so resistant to the Holy Spirit that it will never repent and believe in Christ.