Christ says blasphemy against the Holy Ghost will never be forgiven. (Mt 12:31) What kind of sin is this? How do we know if we’ve committed it?
Christ is responding to the Pharisees’ claim that He Himself, as He healed the sick, gave sight to the blind, cleansed the lepers, raised the dead, cast out devils, and preached good news to the poor, was in league with and empowered by Satan. (Mt 12:24) They found no evidence of evil in Him, but despised Him for denouncing their hypocrisy and wickedness.
Rather than acknowledge Christ’s godly power and turn to God, the Pharisees chose to sin grossly against plain fact, and publicly accuse Christ of being demon possessed. (Jn 10:20) This kind of blatant disregard for truth, this level of aggravated insult to the divine being, a deliberate choosing of deception and lies in the face of miraculous, divine revelation, is what Christ is describing. (Jn 15:24)
As a person continues to give themselves over to this level of deception and wickedness, they’re giving themselves over to darkness, to Satan himself, to be captured and ensnared by him. (2Ti 2:26) There comes a point of no return, at some level, from which no one will ever recover, where one’s conscience is seared with a hot iron, such that distinguishing between good and evil is no longer possible. (1Ti 4:2)
This isn’t the kind of sin a child of God can commit. (1Jn 3:9) A person who’s committed to this level of wickedness isn’t going to be worried about it; they’ll scoff at the idea that they’re in danger of hell fire. There will be no fear of God in them, no desire to repent and cease from their pride and wickedness (Mk 15:31); God will have given them up, turned them over to their own way (Pr 1:31), and abandoned them in their sin. (Pr 1:28-29)