Then the Lord said to Moses, “Pharaoh's heart is hardened; he refuses to let the people go. Go to Pharaoh in the morning, as he is going out to the water. Stand on the bank of the Nile to meet him, and take in your hand the staff that turned into a serpent. And you shall say to him, ‘The Lord, the God of the Hebrews, sent me to you, saying, “Let my people go, that they may serve me in the wilderness. But so far, you have not obeyed.” Thus says the Lord, “By this you shall know that I am the Lord: behold, with the staff that is in my hand I will strike the water that is in the Nile, and it shall turn into blood. The fish in the Nile shall die, and the Nile will stink, and the Egyptians will grow weary of drinking water from the Nile.”’” And the Lord said to Moses, “Say to Aaron, ‘Take your staff and stretch out your hand over the waters of Egypt, over their rivers, their canals, and their ponds, and all their pools of water, so that they may become blood, and there shall be blood throughout all the land of Egypt, even in vessels of wood and in vessels of stone.’”
Moses and Aaron did as the Lord commanded. In the sight of Pharaoh and in the sight of his servants he lifted up the staff and struck the water in the Nile, and all the water in the Nile turned into blood. And the fish in the Nile died, and the Nile stank, so that the Egyptians could not drink water from the Nile. There was blood throughout all the land of Egypt. But the magicians of Egypt did the same by their secret arts. So Pharaoh's heart remained hardened, and he would not listen to them, as the Lord had said. Pharaoh turned and went into his house, and he did not take even this to heart. And all the Egyptians dug along the Nile for water to drink, for they could not drink the water of the Nile.
Seven full days passed after the Lord had struck the Nile.
Exodus 7:14-25
We must pause at this point to say something about the mystery of the hardening of Pharaoh's heart. The subject is not without its problems, and it is only by collating all the references throughout the story and examining them as a whole that we can understand their significance. The references are as follows: 3:19; 4:21; 5:2; 7:3, 13; (7:13 should read, with RSV, 'And Pharaoh's heart was hardened', not as in AV); 7:14, 22; 8:15, 19, 32; 9:7,12; 9:34, 35; 10:1 ,20, 27; 11:10; 14:4, 8. From these references we see that three forms of expression are used: 'Pharaoh’s heart was hardened', 'Pharaoh hardened his heart', and 'The Lord hardened Pharaoh's heart'. Certain things can be discerned here. The hardening of the heart in Pharaoh was the inevitable consequence of his refusal to obey God. Then, the three occasions in which it is said that Pharaoh hardened his heart (8:15, 32; 9:34) follow an actual softening of his heart by God. He deliberately hardened himself against God when he might have sought and found His grace. And it is only after this that it is said that God hardened his heart. It is surely not without significance that only in the later stages of this conflict is it said that God hardened his heart (9:12; 10:20, 27; 11:10; 14:8). Whatever therefore may be the mystery of election and predestination lying behind this question (see Romans 9:14-24), it was no arbitrary decree of God, but Pharaoh's own deliberate resistance of the divine will that brought upon him the judicial hardening of heart from God.