There is a great evil pervading our culture. “Of course,” you say, “our nation is being destroyed by politics, or entertainment, or immorality.” Perhaps that is true, but the evil of which I speak is nothing new. Nor is it a danger of which many are aware.
There is a curious process which happens in the physical body, whereby an accumulation of dead skin cells form in an area to protect a portion of the body from excess friction or pressure. The dictionary defines this accumulation of dead skin cells as a callous, which meaning also includes, “insensitive, indifferent, hardened.” 
Hebrews 3:13 reads, “But exhort one another every day, as long as it called ‘today,’ that none of you may be hardened by the deceitfulness of sin.” The surrounding verses tell of Israel’s rebellion in the wilderness, the Lord’s promise that they would not enter into His rest, and the exhortation to Christians to take care lest they also fall away from the living God (Hebrews 3:7-18). The chapter closes with verse 18, “So we see that they were unable to enter because of unbelief.”
Clearly the Israelites did not fail to enter the Promised Land because they did not believe in the existence of a God. It was God who had led them in the fire by night and the cloud by day, God who had provided water and manna and quail for them in the wilderness, and God who had opened the Red sea for them to walk across on dry land. Scripture tells us however, that even the devils believe in the existence of God (James 2:19). What is it, then, that the Israelites failed to believe which cost them the entrance to the Promised Land? If we go all the way back to Genesis 3 and the fall of man, we see that Satan’s temptation of Eve consisted of the lie, “You will not surely die. For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil.” Satan’s claim was that God had lied to Eve about the virtues of the forbidden fruit because God did not want Eve to be like Himself. What he desperately wanted Eve to believe was that God was not good. I believe that this first mention of Satan’s attack tactics is significant, revealing to us Satan’s primary goal towards all of mankind. Thus, the Israelites did not fail to enter the Promised Land because they did not believe in God, but because they no longer believed in the goodness of God. Psalm 78 records the recurring forgetfulness of Israel with respect to the goodness of God. “They forgot His works and the wonders that He had shown them.” “When He killed them, they sought Him; they repented and sought God earnestly. They remembered that God was their rock, the Most High God their redeemer. But they flattered Him with their mouths; they lied to Him with their tongues. Their heart was not steadfast toward him; they were not faithful to His covenant.” “They did not remember His power or the day when He redeemed them from the foe.” For seventy-two verses, David retells the history of Israel, of God’s unceasing mercy, of Israel’s unfaithfulness and unbelief, and of God’s anger and judgment. The nation of Israel had been “hardened through the deceitfulness of sin,” and in ceasing to believe that God was good, were prohibited from entering the Promised Land. Sin had calloused their hearts to the things of God.
The great danger of which I spoke earlier is this same callousness to the things of God. The deceitfulness of sin is hardening our hearts to the wonder of God. We unconsciously cease to believe in the goodness of God. Like a callous, this is an accumulation of deadness which forms slowly, without notice, until we have lost all sensitivity. Yet there are tell-tale signs of this pervading indifference in our hearts. Scripture reveals to us that our mouth is the exhale of our hearts. What is in our hearts comes out of our mouths. Phrases that I have found proceeding from my own calloused heart include: “It’s unbearably hot out today. I wish it were cooler.” “That’s not fair. Why can’t I…?” “I can’t believe…happened to me.” The thoughts from which these phrases stem often revolve around my plan for my life not playing out in real life. When I don’t have the money for the college that I want to go to, I immediately question God’s goodness in allowing me to be placed in the financial circumstances I am in. Surely, if God were good, He would want me to go to a nice Christian school. A lack of contentment on our part reveals a lack of belief in the goodness (and sovereignty) of God. From “big” things like relationships, college, and finances, to “little” things like the weather, inconveniences, and interruptions, God is in complete control of the day by day circumstances of our lives. It is this very realization that allows David to rejoice in the “worst” of circumstances. In the torment and trial of life, David speaks hope to his soul, “Why are you cast down, O my soul, and why are you in turmoil within me? Hope in God; for I shall again praise him, my salvation and my God.” Then in Psalm 119:68, he confirms the immutability of the goodness of God when he states, “You are good and do good; teach me Your statutes.”
Perhaps in evaluating the thoughts of your heart, you have seen that you too are being hardened by the deceitfulness of sin and are forgetting the goodness of God. Hebrews 3:13, as we have already quoted, says that we are to “Exhort one another every day, so long as it is called ‘today,’ that none of you may be hardened by the deceitfulness of sin.” “Remember,” says the Lord. Do not be as the nation of Israel who forgot the goodness of God, who grew dull to the wonders of God. Isaiah 44 and 46 share the common theme of God calling Israel to remember Him. “Remember these things, O Jacob…I formed You…return to Me, for I have redeemed you.” “Remember this and stand firm, recall it to mind…I am God, and there is no other…there is none like me…I will accomplish all I purpose.” Will you choose to remember and give glory to the God who is sovereign and good? “Rejoice, dear friends in the goodness of God, and let not your hearts be calloused.
How precious is Your steadfast love, O God!
The children of mankind take refuge in the shadow of your wings.
They feast on the abundance of Your house
And You give them drink from the river of Your delights.
For with You is the fountain of life:
In Your Light do we see light.
 Callous. Dictionary.com. September 6, 2009. http://dictionary.reference.com/browse/callous
 Hebrews 3:16. ESV Study Bible, English Standard Version. Crossway Bibles (Wheaton, 2008).
 Genesis 3:4-6. ESV Study Bible, English Standard Version. Crossway Bibles (Wheaton, 2008).
 This is a blatant lie, for God created man in His own image (See Genesis 1:27), and the very plan of redemption is centered around renewing the relationship between God and man, redeeming man from His sin so that he could once again enjoy fellowship with God.
 Psalm 78. ESV Study Bible, English Standard Version. Crossway Bibles (Wheaton, 2008).
 Luke 6:45. ESV Study Bible, English Standard Version. Crossway Bibles (Wheaton, 2008).
 Psalm 42:5. ESV Study Bible, English Standard Version. Crossway Bibles (Wheaton, 2008).
 Psalm 119:68. ESV Study Bible, English Standard Version. Crossway Bibles (Wheaton, 2008).
 Isaiah 44. ESV Study Bible, English Standard Version. Crossway Bibles (Wheaton, 2008).
 Isaiah 46. ESV Study Bible, English Standard Version. Crossway Bibles (Wheaton, 2008).
 Psalm 36:7-9. ESV Study Bible, English Standard Version. Crossway Bibles (Wheaton, 2008).