"But I will harden Pharaoh's heart, and though I multiply my signs and wonders in the land of Egypt, Pharaoh will not listen to you." -- Exodus 7:3-4
This verse makes me shudder. Pharaoh was hardened to the message of God. Several passages tell us that Pharaoh hardened his own heart and would not allow the people of Israel to leave Egypt. But Pharaoh's thoughts and decisions were not the primary cause of his hardness of heart. Don't get me wrong; Pharaoh's contribution to his hardening was entirely his responsibility. He acted according to precisely what he wanted to do. God did not force Pharaoh, against his will, to reject Moses' request. But God did cause him to do precisely that! And Pharaoh did it willingly.
God rules the minds of men, either allowing them to think and act according to their fallen nature, or changing their hearts and minds to think and act according to a new nature.
"And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules." -- Ezekiel 36:26-27
According to the New Covenant, God causes his people to carefully obey. How? By giving them a new heart. God fixes our wants; we used to want things according to our fallen natures, but God, in a miracle of new birth, gives us new desires. God is the primary cause of my desiring God.
So why is this sentence so scary? Because God could have hardened my heart.