The Unpardonable Sin

Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation.
- Mark 3:28-29 (KJV)

In short, the unpardonable sin is "completely and utterly rejecting the Holy Spirit".

Ellen White puts it like this:

The sin of blasphemy against the Holy Spirit does not lie in any sudden word or deed; it is the firm, determined resistance of truth and evidence.
- Ellen White, The Faith I Live By, p. 58

No one need look upon the sin against the Holy Ghost as something mysterious and indefinable. The sin against the Holy Ghost is the sin of persistent refusal to respond to the invitation to repent.
- Ellen White, That I May Know, chapter 237, p. 243

It is by the Spirit that God works upon the heart; when men willfully reject the Spirit, and declare Him to be from Satan, they cut off the channel by which God can communicate with them. When the Spirit is finally rejected, there is no more that God can do for the soul....
- Ellen White, Ye Shall Receive Power, p. 36

I highly recommend spending some time on this, because the unpardonable sin is probably not what you think it is.

My people are destroyed for lack of knowledge.
- Hosea 4:6 (KJV)

God is actually quite willing to forgive all of our sins.

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 (KJV)

God doesn't want any of us to perish.

The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.
- 2 Peter 3:9 (KJV)

Repent therefore, and don't let satan trick you into thinking that "it's all over", because it's not.

The excerpt below is from a Bible study guide from It Is Written.

In the passage in Matthew 12 discussing the unpardonable sin, Jesus heals a man who was "demon-possessed, blind, and mute" (Matthew 12). In attributing the miracle to the working of Satan, the Pharisees rejected not only Jesus but also the Holy Spirit, who was present to bring conviction to their hearts. When an individual refuses to surrender to the conviction of the Holy Spirit, that person is rejecting the Holy Spirit. While God in His goodness continues to bear with resistant people, there comes a time when those who harden their hearts against conviction are left to their own devices. Having rejected the Holy Spirit, they have committed the unpardonable sin. The reason the unpardonable sin is not forgiven is because it is not confessed. If a sin remains unconfessed, the sin in question becomes the unpardonable sin. Please note: There is no need to worry that any sin committed in the past and not confessed is the unpardonable sin. The unpardonable sin represents a continual rejection of the Holy Spirit's pleading - how long until the unpardonable sin is committed is not known. What is known is that it is neither wise nor safe to say "no" to the Holy Spirit. The only right course of action is to surrender when God's Spirit speaks, and to enjoy the peace and blessing of a union with the God of heaven.
- It Is Written Bible Study Guides

The Holy Spirit is guiding us home, let's not grieve Him.

And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption.
- Ephesians 4:30 (KJV)

Further Study

Check out these resources:

For a full-on Bible study on this topic (two sermons) please click here. For part 2, please click here.