The Unpardonable Sin is a Deliberate, Willful Choice and Rejection of God the Father and Jesus Christ, and a Repudiation of the Forgiveness of Sins and the Gift of Salvation Through Jesus' Sacrifice and His BloodThe unpardonable sin is a deliberate and calculated renunciation of God. It is not an accidental sin, nor a sin under temptation, nor a sin through weakness. But the unpardonable sin is a knowing, determined, calculated, premeditated, willful decision and choice arrived at by rejecting any pangs of conscience, leading of the Holy Spirit, pleadings of the Word of God, or ministers and brethren. It is a willful, determination to reject the gift of repentance, and hostile contempt for the commandments of God, the Spirit of God, a willful repudiation of the sacrifice of Jesus Christ and His blood for the forgiveness of sins and the salvation of God. Those who have committed the unpardonable sin have no regrets, no sorrow and no conscience that they have turned their backs on God the Father and Jesus Christ, and the Word of God. It is impossible to renew them to repentance—it is impossible for the Holy Spirit to move them to repentance, for others to move them to repentance, or to move themselves to repentance. They have seared their consciences with a hot iron and there is no power—human or divine—that is able, or has the ability to renew them to repentance. Their choice is final, their judgment is final and God's judgment against them is final.
"In Hebrews the characteristic terms for sin that display contempt for God are compounds of para, many of which occur nowhere else in the New Testament:
para deigmaizein, to expose, to public humiliation (6:6)
para pikainein, to rebel (3:16)
para pikrasmos, rebellion (3:8, 15)
para rrein, to drift a way, fall away (2:1)
para pherein, to carry away (13:9)
par einenos, being listless (12:12)"
"There is no other repentance than that provided through Jesus Christ. There is no salvation apart from the purification for sins accomplished by the divine Son in the final period of God's redemptive activity (1:1-3). The adunatou adunaton [the impossibility], which is used, absolutely and without qualification in Heb. 6:4, expresses an impossibility because the apostate [has] repudiated] the only basis upon which repentance can be extended. To repudiate Christ is to embrace the 'impossible.' " (Lane, Word Biblical Commentary, vol. 47A, p. 142).
The Human ConscienceThe human conscience is an inner creation of God in all men and women—part of the heart and mind. When anyone sins there is an automatic sense of guilt. The worse the sin the greater the guilt and hence, dread. "Conscience is that facility within us which decides [makes us conscious of actions and thoughts] as to the moral quality of our thoughts, words and acts. It gives consciousness of the good of one's conduct or motives, or causes feelings of remorse at evil-doing, but its action is involuntary. A good conscience is one which has no feeling of reproach against oneself, does not accuse oneself of willful wrong" (Cruden's Concordance, definition of conscience).
Warning1) Description of the apostateHeb. 6:6"fallen away"
"crucifying the Son of God"
"publicly holding Him in contempt"
Heb. 10:26, 29"willfully go on sinning"
"trampled underfoot the Son of God"
"has regarded the blood of the
covenant, with which He was
sanctified, as an unholy thing"
"has scorned the Spirit of grace"
2) Prior ExperienceHeb. 6:4-5"once enlightened"
"personally obtained the heavenly gift"
"became partakers of the Holy Spirit"
"have tasted the good Word of God, and the powers of the world to come"
Heb. 10:26, 29"after receiving the knowledge of the truth"
"was sanctified with the blood of the covenant"
3) Impossibility of RenewalHeb. 6:4, 6"For it is impossible ... to renewthem again unto repentance"
Heb. 10:26"there is no longer any sacrifice for sins"
4) ExpectationHeb. 6:6, 8"being cursed, the end of which is for burning"
Heb. 10:27, 29, 31"terrifying expectation of inevitable judgment"
"much worse punishment"
"a fearful thing to fall into the hands of the living God"
Comfort1) BasisHeb. 6:9Appeal to "better things"
Heb. 10:32Appeal to "remember the earlier days"
2) Past Experience as ChristiansHeb. 6:10"work, and the labor of love"
"have served the saints"
Heb. 10:32, 33, 34"endured much conflict in your sufferings"
"companions of those who were enduring the same things"
"showed compassion to me in my bonds"
"gladly endured the plunder of your possessions"
3) Present ResponsibilityHeb. 6:11,12"demonstrating the same diligence"
"be imitators of those with faith and steadfast endurance"
Heb. 10:35, 36"do not cast away your confidence"
"need to have endurance"
4) IncentiveHeb. 6:11,12"the full assurance of the hope until the end"
"inherit the promises"
Heb. 10:35, 36"great reward"
"may receive the promise"
This summary is taken from CBCG.org website.