Q: “I will harden his heart so that he will not let the people go.” – Ex 4:21. Do we know why God hardened Pharaoh’s heart?
A: God’s words here in 4:21 are prophetic. Eventually, god will harden Pharaoh’s heart.
But as the account of the 10 Plagues plays out, what we see is that Pharaoh hardens his own heart against the Lord’s warning time and time again (see Ex 7:14, 8:15, 8:32, 9:7, 9:34)
God is a God of patient and steadfast love. But he is also a God of perfect justice. He cannot (and will not) be mocked. When a person repeatedly hardens their heart to God and his Word, the sad result is that God hardens that person’s heart making repentance impossible.
We should also note that we only know of this situation with Pharaoh because of God’s revealed Word. We can never know with certainty today if someone has been hardened by God- so we try to win the soul of every person.