A Hardened Heart
Right now, God, is talking to you (spiritually speaking). God says, "Are you sure you want to do that?", etc.
When God stops talking to your spirit, it is said that your heart will be hardened, e.g. handed over to sin.
Wherefore (as the Holy Ghost saith, To day 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 alway err in their heart; and they have not known my ways. So I sware 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.
- Hebrews 3:7-13 (KJV)
If there is a strong desire to sin, God will ultimately give up, and as a result there will be a hardened/sinful heart.
This is what happened to Pharaoh (from Egypt).
But the LORD hardened Pharaoh's heart, so that he would not let the children of Israel go.
- Exodus 10:20 (KJV)
God gave Pharaoh the choice to "hear His voice", or to continue sinning (and as a result harden his heart). Pharaoh chose to continue sinning, and God was forced to carry out the 10 plagues.
It's important to realize that God did not "set Pharaoh up", but that it was Pharaoh's choice all along.
Just before the Flood, God made it quite clear that He would not strive with man forever.
And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
- Genesis 6:3 (KJV)
During the last days of this world, God will withdraw His Spirit from man, and this whole world will be plunged into spiritual darkness.
And because iniquity shall abound, the love of many shall wax cold.
- Matthew 24:12 (KJV)
I can't even begin to imagine what a world without God's Spirit will be like.
We need God.
Further Study
Check out this article from Juli Camarin, and this article from Defending Inerrancy.