Thread by @DrTomSchreiner

I think the unforgivable sin is apostasy; that is what it means to blaspheme against the Spirit. In this thread I show a few verses that support this notion.

See these verses from Deuteronomy 29:18–21
“Deut 29:18 Be sure there is no man, woman, clan, or tribe among you today whose heart turns away from the LORD our God to go and worship the gods of those nations. Be sure there is no root among you bearing poisonous and bitter fruit.

Deuteronomy 29:19 When someone hears the words of this oath, he may consider himself exempt, thinking, ‘I will have peace even though I follow my own stubborn heart.’ This will lead to the destruction of the well-watered land as well as the dry land.

Deuteronomy 29:20 The LORD will not be willing to forgive him. Instead, his anger and jealousy will burn against that person, and every curse written in this scroll will descend on him. The LORD will blot out his name under heaven,

Deuteronomy 29:21 and single him out for harm from all the tribes of Israel, according to all the curses of the covenant written in this book of the law.

Notice the sin is turning away from the Lord, worshiping other gods. that is the poisonous and bitter root. The Lord will never forgiven the person who turns against him. He or she will be cursed forever.

Hebrews 12.15 clearly alludes to this verse.
"Make sure that no one falls short of the grace of God and that no root of bitterness springs up, causing trouble and defiling many."

This fits with the other warnings in Hebrews: 2:1–4; 3:7–4:13; 5:11–6:12; 10:36–41 and 12:25–29. The sin that can't be forgiven is refusing to trust in Christ (Hebrews 11!). Or as Hebrews 10:26 says there is no forgiveness for those who turn away from Christ's sacrifice.

Blasphemy against the Spirit, the unforgivable sin is no accident. It is forsaking Christ and his cross, Christ and the forgiveness he offers.