18 ¶ 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 which are in Egypt, and see whether they be yet alive. And Jethro said to Moses, Go in peace. 19 And the LORD said unto Moses in Midian, Go, return into Egypt: for all the men are dead which sought thy life. 20 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. 21 And the LORD said unto Moses, When thou goest to return into Egypt, see that thou do all those wonders before Pharaoh, which I have put in thine hand: but I will harden his heart, that he shall not let the people go. 22 And thou shalt say unto Pharaoh, Thus saith the LORD, Israel is my son, even my firstborn: 23 And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.
See the comments on Exodus 2:16-22 about the two names of Jethro/Reuel. He does not tell Jethro what he has seen or heard or what his mission truly is, just to go back to see if his brethren yet live. God encourages him that those that wanted his death are no longer living. So, he took his wife and sons and began the journey back into Egypt. It must have been with a lot of emotion, self-doubt, and perhaps, not a little fear. God not only tells him to perform the wonders but that they will not make the Pharaoh let his people leave. In fact, God tells Moses to say to Pharaoh that the people of Israel are collectively God’s son, His firstborn. And, if Pharaoh doesn’t let Israel leave, God will kill even Pharaoh’s eldest son.
Verse 22 is an important prophetic verse. There are other verses by cross-reference to compare.
Hosea 11:1 ¶ When Israel was a child, then I loved him, and called my son out of Egypt.
That verse is applied to Christ by the Holy Spirit in;
Matthew 2:15 And was there until the death of Herod: that it might be fulfilled which was spoken of the Lord by the prophet, saying, Out of Egypt have I called my son.
Then there are these verses;
Proverbs 30:4 Who hath ascended up into heaven, or descended? who hath gathered the wind in his fists? who hath bound the waters in a garment? who hath established all the ends of the earth? what is his name, and what is his son’s name, if thou canst tell?
Malachi 3:16 Then they that feared the LORD spake often one to another: and the LORD hearkened, and heard it, and a book of remembrance was written before him for them that feared the LORD, and that thought upon his name. 17 And they shall be mine, saith the LORD of hosts, in that day when I make up my jewels; and I will spare them, as a man spareth his own son that serveth him. 18 Then shall ye return, and discern between the righteous and the wicked, between him that serveth God and him that serveth him not.
These verses point backwards to Israel and forwards to Christ and His body on earth, the church. An interesting point is made under the Law regarding this as well. In prophetic terms, the following verse is one of the foundations for showing that God is not done with Israel, lest the church, Christ’s body on earth, be swelled up in its thinking.
Deuteronomy 21:15 ¶ If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated: 16 Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn: 17 But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.
So, God has given specific instructions to Moses to say a specific thing to Pharaoh, a warning that if he does not let the people go God will kill his eldest son. Moses will be warned personally about this judgment probably due to his doubts and hesitation. Connect this verse 23 with what comes next. The duty and place and heritage of the eldest son was discussed in the comments on Genesis. There was no greater person in a family besides the father himself. To lose the eldest son would be devastating. To lose him as a consequence of God’s wrath cannot be imagined. Apparently, Moses has not committed himself to the sign of circumcision for his own child of a Midianite women required for God’s people. This was the first sign of a Hebrew on his part that he would follow Jehovah God. First, God argued with him, encouraged him, offered alternatives, and gently led him. Now, though, Moses must be shown that God means business.