1. Unbelief is a terrible sin.  It is the sin that sends sinners to hell.
2. “And he said unto them, Go ye into all the world, and preach the gospel to every creature.  He that believeth and is baptized shall be saved; but he that believeth not shall be damned” (Mark 16:15, 16).
3. “He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God” (John 3:18).
4. “He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him” (John 3:36).
5. Many other Scriptures could be cited, showing the danger of unbelief.  Those who do not believe in Jesus will spend eternity in hell.
6. But unbelief affects Christians as well as the unsaved.
7. Hebrews 10:26 says, "For if we sin wilfully after that we have received the knowledge of the truth..."
8. Hebrews 3:12 says, “Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.”
9. There are several other similar warnings in the epistle to the Hebrews.
10. A.T. Pierson said this about unbelief, "You may lose your life's opportunities and privileges, and even God cannot give them back; for He cannot restore the lost hour, the wasted year, the misspent life. What unbelief has forfeited, no power can give back."
11. The Bible does not teach that a believer can lose his salvation.  But the Bible does teach that a Christian can lose his joy, lose his blessing, lose his peace of mind, lose his family, and lose his reward.
12. The way to avoid backsliding is to stay close to the Lord every minute, every hour, and every day. Hebrews 3:13 says, “But exhort one another daily…” Not just on Sunday morning, but “daily.”
13. Some people behave like Christians for a few hours on Sunday, but the rest of the week they behave just like unbelievers.
14. Hebrews 3:13 says, “But exhort one another daily…” A similar statement is found in Hebrews 10:25, “Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.”

I. UNBELIEF REJECTS THE WORD OF GOD (10:26).

1. Hebrews 10:26 says, "For if we sin wilfully after that we have received the knowledge of the truth..."
2. First John 5:10 says, “He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son.”
3. The "record" is God's Word, the Bible.
4. To not believe in the Bible is to make God a liar!  Unbelief is a terrible sin!
5. Oftentimes the Word of God is heard, and it is received, but it is not believed and it is not obeyed.
6. If the Word of God is not appropriated by faith, unbelief makes void the Word of God.
7. This unbelief has a hardening affect. Frequent exhortations will protect us from being “hardened through the deceitfulness of sin” (3:13).
8. Note the repetition – “harden,” “hardened,” etc. (3:8, 13, 15; 4:7).  Hardening -- both physically and spiritually -- happens gradually.
9. Because sin isn’t always punished right away, sinners do not believe what the Bible says about the judgment of God.
10. Ecclesiastes 8:11 says, “Because sentence against an evil work is not executed speedily, therefore the heart of the sons of men is fully set in them to do evil.”
11. Sinners begin to despise God’s patience, mercy, and longsuffering, and their hearts get harder and harder. This hardening process also affects Christians. I have seen Christians harden their hearts.  They bristle and frown when you try to correct them.
12. They make excuses for their backsliding, etc.  They point to others who have sinned and say, “Look at him” or “Look at her.”
13. Backsliding is usually very gradual. God wants us to be “stedfast unto the end” (3:14).
14. Many Christians start out well, but unfortunately they do not finish well.  Notice the words in Hebrews 3 – “firm unto the end” (3:6); “stedfast unto the end” (3:14).
15. The Bible speaks often of a hard heart. The “hardening of the heart” is a spiritual disease.  It is similar to the stiff neck.  Once a person adopts a fixed attitude of rebelliousness and disobedience, his heart gets hard to the things of God.
16. Donald Guthrie wrote, “A hardened state of mind becomes impervious to God’s voice and leads to increasing ignorance of his ways, not because God does not want to make them known, but because the hardened mind has no disposition to listen.”
17. F.W. Krummacher said, "For unbelief is the occasion of all sin, and the very bond of iniquity. It does nothing but darken and destroy. It makes the world a moral desert, where no Divine footsteps are heard--where no angels ascend and descend--where no living hand adorns the fields, feeds the fowls of heaven, or regulates events. Thus it makes nature, the garden of God, a mere automaton; and the history of Providence a fortuitous succession of events; and man, a creature of accident; and prayer, a useless ceremony."

II. UNBELIEF MAKES VOID THE CROSS OF CHRIST

1. One of the saddest statements in the Bible is here in Hebrews 6:6 -- "seeing they crucify to themselves the Son of God afresh, and put him to an open shame."
2. And Hebrews 10:29, "Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing."
3. It is interesting to note that in Hebrews 10:29, the author (probably the apostle Paul) changes from the pronoun "we" (10:26) to "he" (10:29).
4. This indicates that a true believer could not be guilty of trampling under foot the Son of God, and counting the blood of the covenant an unholy thing (10:29).
5. And yet it does say, "wherewith he was sanctified" (10:29).
6. The word "sanctified" means "set apart."  There is a sense in which lost people are sanctified, that is set apart, by certain Gospel privileges.
7. For example, First Corinthians 7:14 says, "For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy."
8. "Holy" and "sanctified" in the sense that they are set apart by God and have certain advantages other sinners do not have.  
9. That is why they will suffer an even great punishment in hell if they die in their sins (cf. Matthew 11:20-24; Luke 12:48).
10. Our Lord rebuked the scribes and Pharisees, and called them "hypocrites," because they had so much light and they rejected that light.  He said to them, "Ye shall receive the greater damnation" (Matthew 23:14).
11. It is a terrible sin to know the truth and reject the truth!  The book of Hebrews teaches that to reject the Lord Jesus Christ is to crucify to Him afresh (Heb. 6:6).
12. It is to be guilty of trampling under foot the Son of God, and counting His precious blood "an unholy thing" (10:29).
13. Over the years, I have seen unbelievers take a Gospel tract and throw it on the ground, only to be stepped on.  It is a terrible thing to do -- but how much worse to do that the Lord Jesus Christ Himself (10:29)!
14. How rude!  How disrespectful!  How insulting! How wicked!

III. UNBELIEF DOES DESPITE TO THE SPIRIT OF GRACE

1. "Done despite" means "to insult." This literally means to insult the gracious Holy Spirit (10:29b).
2. To do despite unto the Spirit of grace means to reject the Gospel, and to reject the Lord Jesus Christ, and to reject the Bible, and to reject God's offer of salvation.
3. Our Lord said in Matthew 12:31, "Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men."
4. Our Lord said in John 16:8 and 9, that when the Holy Spirit is come, "he will reprove the world of sin, and of righteousness, and of judgment: Of sin, because they believe not on me."
5. Unbelief is the crowning sin in God's sight, and unbelief does despite unto the Spirit of grace.
6. There are certain people who attend services like this, and the Holy Spirit convicts them of their sin of unbelief, but they ignore the Holy Spirit (Hebrews 10:29).
7. This is the only time in the New Testament that the Holy Spirit is called "the Spirit of grace."  He is called the Spirit of grace once in the Old Testament, in Zechariah 12:10.
8. H.A. Ironside said, “The Holy Spirit delights to magnify Christ and to exalt His work. To refuse His testimony is to do despite unto the Spirit of grace” (Hebrews).

CONCLUSION:

1. People do not consider unbelief a serious sin, like murder or stealing or homosexuality, etc.  They consider unbelief a trifling sin, a small insignificant sin, when in fact it is a horrible sin (Heb. 10:29-31).
2. Some people are so confused and so deceived they even think unbelief is a good thing -- to them it shows that a person is an open-minded freethinker.
3. Some even mistake unbelief as a sign of humility.  I remember a few years back when a reporter asked Cardinal John O'Connor if he was expecting to go soon to heaven.  Mr. O'Connor said he did not know if he was going to heaven!
4. But the Bible says, "These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God" (I John 5:13).
5. To not believe is the greatest of sins. Unbelief is a wicked sin, deserving of eternal punishment in hell. Revelation 21:8 refers to the various sinners that will be cast off into the lake of fire and brimstone -- the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars -- and in that terrible group of ungodly sinners are the "unbelieving."