What is the unforgivable sin against the Holy Spirit?

I heard about this “unforgivable sin” of blaspheming the Holy Spirit. Supposedly, in Mark 3:29, Matt 12:31-32, and Luke 12:10. Does that mean once someone renounces their faith, they can never return?

The Church has said that the sin against the Holy Spirit is to refuse God’s mercy and forgiveness.

Catechism of the Catholic Church:

1864 “Whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin.” 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.

An atheist who repents receives God’s mercy and forgiveness.

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