Many people feel that they can’t come (back) to Christ because they’ve “committed the unforgivable sin.” People think that this sin is undefined or is one of the many sins they have done. This is a lie from Satan. So where does the concept of “unforgivable sin” come from?
Jesus said in Mark 3:28–29, “Truly I say to you, all sins shall be forgiven the sons of men, and whatever blasphemies they utter; but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin.” (NASB) So, “blaspheming the Holy Spirit” is an unforgivable and eternal sin…but what does that even mean? The answer is given in Hebrews.
Hebrews 10:26–29 (NASB)
 For if we go on sinning willfully after receiving the knowledge of the truth, there no longer remains a sacrifice for sins,  but a terrifying expectation of judgment and the fury of a fire which will consume the adversaries.  Anyone who has set aside the Law of Moses dies without mercy on the testimony of two or three witnesses.  How much severer punishment do you think he will deserve who has trampled under foot the Son of God, and has regarded as unclean the blood of the covenant by which he was sanctified, and has insulted the Spirit of grace?
While the first part might at first sound like apostacy (falling away from God after receiving Christ), it’s not. What it’s saying is what Paul said in Acts 4:12, “There is salvation in no one else; for there is no other Name under heaven that has been given among men by which we must be saved.” Search as you may, and believe what you want, but there is no salvation apart from Christ.
John 14:6 (NASB)
Jesus said* to him, “I am the way, and the truth, and the life; no one comes to the Father but through Me.
The second part of this passage is where we get to the “blaspheming the Holy Spirit.” By choosing to refuse the Holy Spirit’s call to repent of your old ways and turn to Christ…the only One who can make you clean before the Father…the only One who can wash away EVERY stain with His holy blood…you “insult the Spirit of Grace.”
Philippians 2:9–11 (NASB)
 For this reason also, God highly exalted Him, and bestowed on Him the name which is above every name,  so that at the name of Jesus every knee will bow, of those who are in heaven and on earth and under the earth,  and that every tongue will confess that Jesus Christ is Lord, to the glory of God the Father.