Hebrews 3:12 Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.The sin of unbelief. It's the only sin that will damn a man to hell – the truly unpardonable sin. When Jesus gave His life on the cross, was buried, and rose again, He paid the price for the sin of all mankind. "The blood of Jesus Christ…cleanseth us from ALL sin" (1 John 1:7), and there is no sin outside the bounds of God's forgiveness. A man can commit any sin in the book – and be forgiven – when he repents and turns to the Savior (be careful not to pervert the amazing extent of God's grace as a license to go commit any of those sins). But a man who does not believe on Jesus Christ finds no such forgiveness – no matter what his sin, no matter what his level of religious devotion, etc.
The sin of unbelief, like pride, is also a root sin. Hebrews 3:12 says that ANY departure from God ultimately stems from an evil heart of unbelief. Why is it that we as Christians – saved and on our way to heaven by the grace of God – fail to do what we should? In some way, at some point, it's a failure to believe the word of God. Why is that we as Christians – cleansed and forgiven and eternally secure – do what we shouldn't? It's because we fail to believe the Bible.
"Lord, increase our faith" (Luke 17:5). We know what God's word says to do. We know what God's word says not to do. And the Bible is very clear about the consequences we can expect either way.
Hebrews 3:13 But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
But sin steps in and deceives us. Somehow it convinces us that God didn't mean what He said; that the consequences won't be all that bad; that we've got this under control and can stop whenever we want; that it'll be worth it. LIES!
Let's make up our minds that God means what He says in His book; that things will really turn out the way He said they would; and we'd be best off simply following Him.
Galatians 6:7-8 Be not deceived; God is not mocked: for whatsoever a man soweth, that shall he also reap. For he that soweth to his flesh shall of the flesh reap corruption; but he that soweth to the Spirit shall of the Spirit reap life everlasting.