Institutes of the Christian Religion (1559) - John Calvin
Book 3 - How We Receive the Grace of Christ
Chapter 3 - Our Regeneration by Faith: Repentance
Sections 21-25 - Sins for which there is no repentance or pardon
Section 21 - Repentance as God's free gift
God gives it - Acts 11:18; 2 Cor 7:10; 2 Tim 2:25-26; Eph 2:10.
Without the Spirit working, our hearts remain hard.
Isa 63:17; Heb 6:4-6; 10:26, 29.The unforgiveable sin of Matt 12:31-32; Mark 3:28-29; Luke 12:10 relates to this, and "cannot be ascribed to weakness," but only to stubborn and rebellious evil.
Section 22 - Unpardonable sin
... is to knowingly "resist God's truth." Matt 12:32.
Calvin disagrees with Augustine here!
Reprobates rage against God, but when those whom the Spirit of truth has touched do so, it is the unforgiveable sin. Acts 6:10; Matt 9:34; 12:24; 1 Tim 1:13.
Section 23 - How the impossibility of "second repentance" is to be understood
Heb 6:4-6; 10:26; 1 John 2:19
You don't just lapse into this sin, almost accidentally;
it is "choking the light of the Spirit with deliberate impiety."
Section 24 - Those who cannot be forgiven are those who cannot repent
This is consistent with God's mercy;
He doesn't refuse pardon to those who repent - Ezek 18:21-22.
But those who choke the Spirit will not repent.
When they cry, and God doesn't hear, that cry is not true repentance.
Heb 12:17; Zech 7:13.
Section 25 - Sham repentance and honest repentance
There is a temporary, fake repentance that sometimes brings temporary mercy from God.
Ahab - 1 Kings 21:27-29. Esau - Gen 27:28.
These are examples to the godly, both of God's ready mercy and His ultimate condemnation when we fail to repent sincerely.