The unforgivable sin is blaspheming the Holy Spirit (Matt. 12:22-32).  Jesus was casting out demons by the miraculous power of the Holy Spirit (v. 28), but the Pharisees’ hearts were so hardened against Him that they refused to attribute His miracles to the power of the Spirit, instead blaspheming the Spirit by attributing His work to Satan (v. 24).

The reason their sin is unpardonable is due to the hardness of their hearts which made it impossible for them to repent, accept Christ, and be forgiven (cf. Heb. 6:4-6).

While miracles done by men via the Holy Spirit’s power have ceased today (1 Cor. 13:8-10), it is still possible to have a heart so hardened that one rejects the Spirit-inspired Word of God and thus fails to repent, turn to Christ, and be subsequently forgiven (Heb. 10:26-31; cf. 6:4-6; 1 John 5:16-17; Lk. 8: 5, 12).

This is why it is so important to have open and honest hearts when one receives the message from the Spirit which is God’s Word (Lk. 8:15).