Calvinism and unforgivable sin

John Piper's sermon Beyond Forgiveness: Blasphemy Against the Holy Spirit is the best sermon I've heard on this topic

The way I understand it is, yes there is sin for which there will never be forgiveness, but no there is not sin for which there will never be forgiveness if we repent. To be beyond forgiveness is to be beyond repentance. The implication being that if God removes his Holy Spirit from us, we will not repent, and we never were one of the elect because God accomplishes what he intends to do.

Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen. For this reason God gave them up to dishonorable passions (Romans 1).

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So there can be a point where God says, "Enough," lifts his grace, and allows us to hang ourselves with our own wickedness. That's why David cried out, "Take not your Holy Spirit from me... and uphold me with a willing spirit" (Psalm 51).

JMC309 said:

If you look in the Christian advice section and stick aroujd for a bit, it is almost guaranteed that someone is going to ask about this. It's a common worry. People believe that they are beyond salvation on the basis of Matthew 12, Mark 3, Luke 12:10, Hebrews 6:4-8, Hebrews 10:26-29, 2 Peter 2:20 and 1 John 5:16. I struggled with this issue for some time and incidently it was this struggle that brought me to verses like Ephesians 2:8-9 and 2 Timothy 2:25 and so to more Reformed doctrines of salvation. My question is: how are we to interpret these passages given that we hold to the principle of taking the literal meaning of the passage as fundamental? How does it square with perseverance of the saints?

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It squares well. Short synopsis:
Matt 12, Mark 3, Luke 12: the unforgivable sin is considering the Holy Spirit to be the Devil. And no, nobody who will be saved will speak so against the Holy Spirit. Many consider the Spirit of God to be at work in people from early on in their lives, before they express faith. The Spirit's work precedes the person's faith. There's some question whether birth by the Spirit long precedes faith, but that's another issue. The Spirit doesn't just instantly "get aquainted" with the person He's going to regenerate. He knows the person from the foundation of the world.

Heb 6:4-8: while there are multiple explanations, I think the Apostle is obvious by his silence: the person may have all the trappings, but he doesn't have faith.

Heb 10:26-29: "full knowledge" of the truth is not relying on that knowledge -- and reliance is the essence of saving faith.

2 Pt 2:20: there are multiple explanations, but Christ is Lord of the Universe, not simply the people He saves. The Crucifixion was about more than salvation. It's also about condemnation, a new nation, a new family, a new community.

1 Jn 5:16 a few explanations, but this could simply be a sin leading people to die physically, as in 1 Cor 11:30. Or another view, it may be a brother in the church. There are people in the church that John is writing to, who are "in it, but not of it." (1 Jn 2:19) John could be separating the church away from such people (they were essentially Gnostics) -- and no, they weren't saved in the first place even though the church had considered them members, and thus they were called "brothers".