Many have asked about what is often called the unpardonable sin.
Christ mentioned this unforgivable sin in Mark 3:28-29 Mark 3:28-29  Truly I say to you, All sins shall be forgiven to the sons of men, and blasphemies with which soever they shall blaspheme:
 But he that shall blaspheme against the Holy Ghost has never forgiveness, but is in danger of eternal damnation.
American King James Version×, where it is described as blasphemy against the Holy Spirit. It had to do with denying the authority and power of God. Mark explains that Christ was responding to an accusation from His detractors, who said, "He has an unclean spirit" (Mark 3:30 Mark 3:30Because they said, He has an unclean spirit.
American King James Version×). They meant that His authority came from Satan, not from God. Although these accusers knew that Christ was the Son of God, they accused Him of being a fraud. Christ warned them that they were in danger of committing a sin for which there was no forgiveness.
What is the fate of one who does not repent of this sin? The unrepentant will die in a lake of fire at the end of the age (Matthew 10:28 Matthew 10:28And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.
American King James Version×; Revelation 20:14-15 Revelation 20:14-15  And death and hell were cast into the lake of fire. This is the second death.  And whoever was not found written in the book of life was cast into the lake of fire.
American King James Version×). It's not that one sin is more terrible than another is, but rather that any sin we don't repent of will result in death. Hebrews 6:4-6 Hebrews 6:4-6  For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,  And have tasted the good word of God, and the powers of the world to come,  If they shall fall away, to renew them again to repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
American King James Version×and Hebrews 10:26-27 Hebrews 10:26-27  For if we sin willfully after that we have received the knowledge of the truth, there remains no more sacrifice for sins,  But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.
American King James Version×both mention this sin as well, warning us not to "fall away" or to "sin willfully." How does one know if he has crossed the point of no return?
So long as one desires to repent, to get his or her life back in order spiritually, God will always forgive. Sadly, it is possible to come to a state of not caring about and not looking for forgiveness. If there's no repentance, there can be no forgiveness. That's when a person is in danger of the unforgivable sin. A person can come to that place either by simply drifting away or by sharply turning away from God in angry hostility.
At any time, God can and will forgive the sin of departing from Him, if the sinner wants forgiveness and genuinely repents. God inspired John to write, "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (1 John 1:9 1 John 1:9If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
American King James Version×). Clearly, God will extend mercy to all who sincerely repent and change—regardless of their sin. Christ's words, coupled with the warnings in Hebrews, are sobering. Nonetheless, we should hold fast to the hope that God will grant us repentance when we turn to Him.
Let's broaden the subject to consider a related aspect. Quite apart from the unpardonable sin, many sincere religious people do not understand that sin is the transgression of God's law (1 John 3:4 1 John 3:4Whoever commits sin transgresses also the law: for sin is the transgression of the law.
American King James Version×). Most churches teach and practice a theology much different from the way of life followed by the early New Testament Church of God.
For more understanding, please read our free Bible study aid booklet The Road to Eternal Life.