Wherefore (as the Holy Ghost saith, today if ye will hear his voice, harden not your hearts, as in the provocation, in the day of temptation in the wilderness: when your fathers tempted me, proved me, and saw my works forty years.  Wherefore I was grieved with that generation, and said, They do always err in their heart; and they have not known my ways.  So I swore in my wrath, They shall not enter into my rest.) Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin. For we are made partakers of Christ, if we hold the beginning of our confidence steadfast unto the end; wh ile it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation. For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses. But with whom was he grieved forty years? Was it not with them that had sinned, whose carcasses fell in the wilderness?  And to whom swore he that they should not enter into his rest, but to them that believed not? So we see that they could not enter in because of unbelief.  Hebrews 3:7-19

There are those who, despite having heard the Gospel repeatedly, continue to reject GOD's appeal to them. There are also believers who once faithfully served the Lord in many capacities, and yet have drifted away. Both conditions describe a hardened heart.

An individual with a hardened heart is someone who is unimpressed by the Word of GOD and is, or becomes, insensitive to His voice. This person is indifferent, unyielding, resistant, and calloused when it comes to spiritual matters. The evident trait associated with this spiritual condition is a refusal to listen or obey. The Scriptures give several examples of people with hardened hearts.

Pharaoh's heart was hardened over and over again (Exodus 7-11).

And the LORD said unto Moses, When thou goest to return into Egypt, see that thou do all those wonders before Pharaoh, which I have put in thine hand: but I will harden his heart, that he shall not let the people go.  Exodus 4:21

The Children of Israel hardened their hearts at Meribah (Exodus 17:2-7)

Harden not your heart, as in the provocation, and as in the day of temptation in the wilderness:  Psalm 95:8

The disciples did not understand (Mark 8:17-38).

For they considered not the miracle of the loaves: for their heart was hardened.  Mark 6:52

How in the world does the heart become hardened?  Surprising to some, it is a process for which we are personally responsible. Unbelief is the root cause.

Afterward he appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heart, because they believed not them which had seen him after he was risen.  Mark 16:14

Do you ever have a problem with unbelief?   If you do. you can seek GOD's remedy, He will prevent hardness of heart from setting in.

And straightway the father of the child cried out, and said with tears, Lord, I believe; help thou mine unbelief.  Mark 9:24

If we persist in stubborn unbelief, however, the result will be rebellion against GOD.

For rebellion is as the sin of witchcraft, and stubbornness is as iniquity and idolatry. Because thou hast rejected the word of the LORD, he hath also rejected thee from being king. 1 Samuel 15:23

As we begin to entertain thoughts that are displeasing to GOD, it doesn't take much before we act on them. What may start as a single incidence of sin can all too easily be repeated and turn into a habit. Our conscience will initially sound an alarm, but as we persist in our sinful attitude and behavior, we turn a deaf ear to that inner warning signal and eventually even justify our unholy decisions.

For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another; Romans 2:14-15

When we become increasingly entrenched in that manner of thinking and behavior, it is not uncommon to resist reproof and become insensitive to reason and instruction from GOD's Word.

What are things that cause a hardening heart in our lives?  There are several.

Preoccupation with wrong priorities, such as worldly desires and goals, will harden hearts.

But seek ye first the kingdom of God, and his righteousness; and all these things shall be added unto you. Matthew 6:33

Neglect of private devotions and public worship will harden hearts.

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.  Hebrews 10:25

And the deceitfulness of sin, which falsely promises fulfillment, will most surely harden hearts.

Let no man say when he is tempted, I am tempted of God: for God cannot be tempted with evil, neither tempteth he any man: but every man is tempted, when he is drawn away of his own lust, and enticed.  Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death.  Do not err, my beloved brethren.  James 1:13-16

Hardening of the heart can cause you to fall away and miss GOD's best for you. So how can you protect yourself?  To keep from a hardened heart be sure to conduct regular "checkups." There are a couple of things that help in this process.

First, Pray for GOD to search your heart and reveal any areas of resistance to Him.

Search me, O God, and know my heart: try me, and know my thoughts: and see if there be any wicked way in me, and lead me in the way everlasting.  Psalm 139:23-24

Secondly, ask someone, a trusted believer in Jesus, to watch you for signs of a hardening heart.

And I myself also am persuaded of you, my brethren, that ye also are full of goodness, filled with all knowledge, able also to admonish one another.  Romans 15:14

GOD is thinking about every one of us all the time. If we were to add up his thoughts of each of us, they would outnumber the grains of sand. As big as he is, our GOD is also intensely personal. He is intimately concerned with the condition of our hearts. Satan always wants us to doubt the truth, and to look anywhere other than to GOD from where all truth comes. For when we begin to doubt and not believe what GOD says, we also begin to harden our heart. When we hold fast to our faith and believe GOD, we keep our hearts softened so that He can continue to mold us and make us into the person He created us to be.