Exodus 4:21-23 - I will harden his heart

4:21

God then issues what might seem at one level a rather discouraging words. He was to perform all the miracles that God enabled him to do, but then adds that God would harden Pharaoh’s heart. So Pharaoh would not respond well to the miracles. Now this gives some people a problem, one alluded to by Paul in Romans 9:14-24. First we should note that the Bible has no problem with this! We think that God ordaining something and human responsibility are somehow opposed to each other. At a human level they are. If I was going to ordain something I would have to make sure that everyone did exactly what they were told, I could not allow any room for free will. But it is not me, nor any other man, ordaining these things, it is God. God is eternal, we are not. So any problem you see between the sovereignty of God and human responsibility is utterly illusory. 

4:22,23

In telling Moses what would happen God is ensuring that he does not get discouraged when Pharaoh is as stubborn as he proves to be. In John 16 (and elsewhere) Jesus warns us that we will face trouble and will be hated by the world. We should not get discouraged by this, it is all part of the plan. God then tells Moses how He will overcome Pharaoh’s stubbornness. The final plague, the plague of the death of the firstborn will force Pharaoh to let them go. Israel is also referred to as His firstborn.