“There is a sin which is not unto death,
Those who commit this sin do not walk after the Spirit in the footsteps of our Lord. They are “walking after their own lusts [desires]; and their mouths speak great swelling words, having men's persons in admiration because of advantage.” They are men-pleasers because of their self-seeking. (Jude 11-16)
The Apostle Peter (2 Peter 2:10-22) further declares that these Christians had “escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ, and had become entangled again and overcome.” They are like “the dog returning to his own vomit, and the sow that was washed to her wallowing in the mire.” He compares these to Balaam, who left righteousness to receive earthly riches.
The Epistle to the Hebrews gives two descriptions of these people. In the first (Hebrews 6:4-9), the Apostle points to some who, after tasting of the heavenly gift and the powers of the coming age, after receiving the holy Spirit, fall away into sin. This is not due to unavoidable weakness of the flesh and temptations of the Adversary. But they willingly, knowingly abandon righteousness. These, the Apostle assures us, will not repent. They have hardened their hearts. Therefore, there remains nothing further for them.
In Hebrews 10:26,27,31, the Apostle describes another class, which instead of falling away into a sinful course of life, fall away from faith altogether. Their faith in Jesus once justified them and gave them a relationship with God. It is their willfulness that establishes the seriousness of their sins: “If we sin willfully after we have received a knowledge of the truth [after that we have been favored of God in Christ with wisdom, justification and sanctification] there remains no more sacrifice for sins.”
Once Jesus’ merit covers our sins and we sin with full intention – without any weakness or temptation – that would be a sin unto death – Second Death. These harden their hearts and persist in their sins.
Hebrews 10:29 explains that those who once accepted Christ as their Redeemer, and then debase Christ’s sacrifice by considering His sacrifice ordinary, lose their relationship with God. From this sin, there is no further redemption, no resurrection, no recovery – the Second Death. No wonder the Apostle warns us that we should be careful. God’s only provision for mercy toward sinners is through the redeeming work of Christ Jesus our Lord.