But whoever blasphemes the Holy Spirit has no forgiveness forever, but remains guilty of an eternal sin.”
This verse is one of the most serious warnings in the Gospel. It does not contradict the greatness of mercy in the previous verse, but explains what kind of condition places a person outside the bounds of forgiveness. Blasphemy against the Holy Spirit here refers to stubborn, conscious, and hardened resistance, when, for example, a person clearly sees the work of God’s Spirit and knowingly attributes it to evil. It is not an accidental word, nor a passing fear of some troubled believer, but a deeply hardened state of heart that calls light darkness and darkness light. Such a person does not want to repent, because the very power that should have brought him to repentance is rejected and blasphemed by him. Therefore his condition is unforgivable not because God’s mercy is limited, but because he shuts himself off from that mercy, rejecting its testimony.