They could not enter in because of unbelief

Harden NOT your hearts.

Heb 3:7  Wherefore (as the Holy Ghost saith, To day if ye will hear his voice,
Heb 3:8  Harden not your hearts, as in the provocation, in the day of temptation in the wilderness:
Heb 3:9  When your fathers tempted me, proved me, and saw my works forty years.
Heb 3:10  Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways.
Heb 3:11  So I sware in my wrath, They shall not enter into my rest.)
Heb 3:12  Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.

Take heed…
Lest there be in ANY of you...
An evil heart of unbelief…
In departing from the living God.

Heb 3:13  But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.

Exhort one another DAILY while it is called today-
Lest ANY of you be hardened through the deceitfulness of sin.

Sin can harden us.
Sin can corrupt us.
Sin can do to us what it did to those who did not believe in God's love.
Sin's deceit- that it is harmless, that sin doesn't matter. The acceptance of sin as a part of our life rather than something to be despised, this is deceitfulness. We can't know what sin is except by God's moral law. Take God's moral law out of the picture and you are left with a lot of chaos. And yet some people live within that chaos as if it is a normal way of life, acceptable, and even good.

Sin- transgressing God's moral law in any way cannot be acceptable to us.  Sin's deceitfulness would have us accept it and this is the problem a lot of us are consumed by.  The sins we accept set about hardening our hearts causing us to depart from the living God and His truth.

Deceitfulness of sin.

Sometimes it is easier to see the sin in another's life than in one's own. Is it any wonder we need to exhort one another? That we need to daily involve ourselves in the spiritual warfare going on by recognizing first that we are in a war, second by realizing the weapons of this warfare are often fiery darts of our cherished sins, thirdly by repenting of those sins, seeking forgiveness from our Savior. Don't ever accept sin as being something good, something inevitable, something that is a part of us that we just have to forget about. Even a sin that is committed daily, hourly, minutely MUST be recognized for what it is! We cannot let any sin in our lives become acceptable to us.  Sin will deceive us every chance it gets. Sin wants to become acceptable to us so that we no longer repent of it, no longer call it sin.

How often we tell ourselves lies and do so long enough that we accept those lies as truth. Our reality becomes skewed to fit the lies as truths so that we do not embrace the pain and anguish of our sins. There is a horror in sin, that through that horror Satan will take every opportunity to destroy a person. There is a greater horror in the reality that choosing to live a lie rather than face the horror of sin, doing what you have to do just to stay sane and live your life as best you can, this is the deceitfulness of sin that will destroy our relationship with God because we do NOT fully believe in His love and all that entails, His saving grace. 

We need to EXHORT one another daily.

Exhort-
Strong's: From G3844 and G2564; to call near that is invite invoke (by imploration hortation or consolation): - beseech call for (be of good) comfort desire (give) exhort (-ation) intreat pray.

In many Bible translations the word exhort is translated - encourage-  Encourage one another daily. In others it is comfort, Comfort one another daily.

Heb 3:13  But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.

Exhorting one another so that we are not hardened by sin's deceitfulness- means what exactly? Sin will harden us against God because that is the nature of sin- to separate us from God's love. If I'm exhorting, encouraging, comforting, praying for others daily I'm doing so in such a way that sin won't be deceptive and allowed to harden their hearts. This really means revealing the love of God to others daily, because it is only that love of God that can stave off sin's hardening curse. It's only faith in God to love, to forgive that keeps sin from entrenching us in its hateful cement that threatens to destroy our belief in God.

When our belief in God's ability to love and all that means is gone then our hearts are hardened in unbelief and we will be as lost as they were who were left to die in the wilderness for choosing not to believe.

Heb 3:14  For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
Heb 3:15  While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.
Heb 3:16  For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.
Heb 3:17  But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?
Heb 3:18  And to whom sware he that they should not enter into his rest, but to them that believed not?
Heb 3:19  So we see that they could not enter in because of unbelief.

We must believe, all by the grace of God we must believe.

We must keep the belief in Christ alive and well, a living, breathing belief that would have us recognize that the sins in our lives need to be repented of and forgiven by our Savior.

We are made partakers of CHRIST…
IF we hold the beginning of our confidence stedfast unto the end.

And in the beginning of our walk with Christ we were very repentant of our sins, we were sure of Christ's forgiveness of our sins as we repented and asked to be forgiven. We can never cease to seek forgiveness, we can never stop repenting when sin fills our lives, never!  We must forever believe in our SAVIOR and HIS LOVE to save us.