A sin against the Holy Spirit cannot be forgiven?

I was asked about a verse in the Bible that says, roughly, ‘Sin against Jesus can be forgiven if you seek redemption, but a sin against the Holy Spirit cannot be forgiven’. Does anyone know where I might find that in the Bible, and more importantly, what that means? Thanks for any help.

St.Matthew’s Gospel:12:24-32(Douay-Rheims)The Blasphemy of the Spirit:The sin here spoken of is that blasphemy,by which the Pharisees attributed to the miracles of Christ,wrought by the Spirit of God,to Beelzebub,the prince of devils.Now this kind of sin is usually accompanied with so much obstinacy,and such willful opposing the Spirit of God,and the known truth,that men who are guilty of it,are seldom or never converted;and therefore are never forgiven because they will not repent(Challoner).Ver.32 will not be forgiven in this world or the world to come(St.Augustine and St.Gregory-some sins may be forgiven in the world to come;and consequently,that there is a purgatory. Hope this helps,Rocky.

From the Catechism:
1864 "Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven."There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss.

The way I understand it is that sins against the Holy Spirit all prevent repentance and forgiveness by their very nature, such as despair, presumption, and impenitence.

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