Matthew Henry Commentary Exodus 4:18-23

"And Moses went and returned to Jethro his father-in-law, and said unto him, Let me go, I pray thee, and return unto my brethren that are in Egypt, and see whether they be yet alive. And Jethro said to Moses, Go in peace. And Jehovah said unto Moses in Midian, Go, return into Egypt; for all the men are dead that sought thy life. And Moses took his wife and his sons, and set them upon an ass, and he returned to the land of Egypt: and Moses took the rod of God in his hand. And Jehovah said unto Moses, When thou goest back into Egypt, see that thou do before Pharaoh all the wonders which I have put in thy hand: but I will harden his heart and he will not let the people go. And thou shalt say unto Pharaoh, Thus saith Jehovah, Israel is my son, my first-born: and I have said unto thee, Let my son go, that he may serve me; and thou hast refused to let him go: behold, I will slay thy son, thy first-born." — Exodus 4:18-23 (ASV)

After God had appeared in the bush, He often spoke to Moses. Pharaoh had hardened his own heart against the groans and cries of the oppressed Israelites; and now God, in the way of righteous judgment, hardens his heart against the teaching of the miracles and the terror of the plagues.

But whether Pharaoh will hear, or whether he will forbear, Moses must tell him, Thus saith the Lord. He must demand a discharge for Israel, Let my son go; not only my servant, whom you have no right to detain, but my son. It is my son who serves Me, and therefore must be spared, must be pleaded for. In case of refusal I will slay your son, even your firstborn. As men deal with God's people, let them expect so to be dealt with.