"Let me go", says Moses.
Jethro, the shepherd-priest, gives a very different answer to the one Pharaoh will give: "Go, and I wish you well!" (v18) If only Pharaoh had the Spirit of Jethro!
Pharoah will not let them go and the LORD knows it. In chapter 3:19 He says:
But I know that the king of Egypt will not let you go unless compelled by a mighty hand.
Literally this is a "hand of strength" or a "hard hand".
In Exodus 4, the hand of Moses has been made strong/hard - see the three signs he was given earlier in the chapter and the staff that he now carries in his hand (v20). And so the hardness of the LORD's hand (through Moses) will prove stronger than the hardness of Pharaoh's heart (v21).
It's a battle of between two kinds of strength (hardness). The strong/hard hand of the LORD (probably a title for the Spirit) working through His humble servant (Moses) versus the strong/hard heart of Pharaoh and his mighty host.
But this is not a fair fight. Viewed from the flesh it looks like its unfairly stacked against Moses and the Israelites. But seen from the perspective of the Mighty Spirit of the LORD, Pharaoh is shown to have no power of his own. Even the strength of Pharaoh is a derived strength.
Verse 21 is emphatic: the LORD says "I, even I will harden/strengthen Pharaoh's heart." Even the strength of the LORD's opposition comes ultimately from the LORD Himself! The pretensions of the world's mightiest man are only a parasitic perversion of Christ's own power. And the strengthening that happens is only the handing over of Pharaoh to his own much-desired self-will (cf 3:19).
Exodus 4:21 is the first mention of the hardness of Pharaoh's heart. As events unfold the hardening is spoken of as follows:
Pharaoh's heart was hard/strong (7:13)
Pharaoh's heart was heavy / unyielding (this is also the word for 'glory') (7:14)
Pharaoh's heart was hard/strong (7:22)
Pharaoh made heavy his heart (8:15)
Pharaoh's heart was hard/strong (8:19)
Pharaoh made heavy his heart (8:32)
Pharaoh's heart was heavy (9:7)
The LORD hardened the heart of Pharaoh (9:12)
Pharaoh made heavy his heart (9:34)
Pharaoh's heart was hard/strong (9:35)
The LORD made heavy Pharaoh's heart (10:1)
The LORD hardened the heart of Pharaoh (10:20)
The LORD hardened the heart of Pharaoh (10:27)
The LORD hardened the heart of Pharaoh (11:20)
There is an interplay of Pharaoh's hardening and the LORD's but again we must be clear that the LORD does not hand Pharaoh over to anything which he does not actually want himself. (cf. Rom 1:24,26,28)
And all of this reaches a climax in 14:16-18 where "hand", "harden" and "heavy/glory" are combined:
16 Lift up your staff, and stretch out your hand over the sea and divide it, that the people of Israel may go through the sea on dry ground. 17 And I will harden the hearts of the Egyptians so that they shall go in after them, and I will get glory over Pharaoh and all his host, his chariots, and his horsemen. 18 And the Egyptians shall know that I am the LORD, when I have gotten glory over Pharaoh, his chariots, and his horsemen."
The strengthening of the heart in opposition to the LORD emboldens them to pursue their own glory. But the strength of the LORD's hand is proved when this very thing is turned to the LORD's glory - the salvation of His people.
Here is the power of the cross.
26 The kings of the earth set themselves, and the rulers were gathered together, against the Lord and against his Anointed' 27 for truly in this city there were gathered together against your holy Servant Jesus, whom you anointed, both Herod and Pontius Pilate, along with the Gentiles and the peoples of Israel, 28 to do whatever your hand and your plan had predestined to take place. (Acts 4:26-28)
It is the glory of the LORD to make His enemies serve His saving purposes. The ultimate show of this is the cross, but Pharaoh is a powerful foretaste.
Verses 22 and 23 reveal what's at stake in the battle of these two kinds of strength:
`This is what the LORD says: Israel is my firstborn son, 23 and I told you, "Let my son go, so that he may worship me." But you refused to let him go; so I will kill your firstborn son.'
It's a battle of the sons. One is set to inherit the throne of the world's greatest super-power, the other is a down-trodden slave. But because of the LORD's mighty hand, God's son will inherit all the promises and Pharaoh's son will die under divine judgement.
Of course the very Speaker of these words is the True Son (who is also the Avenging Angel!) and He will die as a Lamb under the divine judgement. God's Son, the LORD Jesus, will suffer as Pharaoh's son so that God's son, Israel will be saved. But one way or another there will be blood. Which is what the next section is about.
In v24-26 we see that Moses has been careless with the bloody sacrament of circumcision. And this matters a great deal to the Lord! He would be the Son - the Seed - cut off as a Lamb under divine judgement. And the people were meant to cut this covenanted promise of salvation into their bodies - to very nearly cut off their own seed in this bloody sign.
Well Moses is about to go and enact this great Old Testament foreshadowing of the cross to save God's son Israel - and he hasn't even put the Old Testament sign of the cross on his own son. The Lord is angry. He will not have Moses enact these gospel prophecies without taking the gospel promises seriously. These signs matter to the Lord and He's angry enough to kill the man He's just commissioned to lead the people!
Thank God for his Midianite wife who performs the emergency procedure and touches his feet with it. And surely the "his" refers to the Lord's feet. The Lord is the great Bridegroom of Blood. He cuts the marriage covenant with His people in His own blood. That's what these signs point towards - the Lord's own blood. It's His bloody death that wins a bride. When Moses and Zipporah own this truth in the sacrament, the Lord lets them alone.
In v27 we see the fulfilment of 4:16, "you shall be as God to him." Moses is God, Aaron is his prophet. And all of Moses' words and deeds will be spoken and performed by his prophet. (Of course this sets us up for Deuteronomy 18:18f).
And so naturally - just as the God of Abraham met Moses on Sinai, now Moses (as God) meets Aaron on Sinai.
They go to the people and pass on the good news:
when they heard that the LORD was concerned about them (lit. "had visited them") and had seen their misery, they bowed down and worshipped.
Isn't that wonderful? The LORD Jesus doesn't just visit us - He sees our misery. This reminds us of those wonderful verbs in chapter 2 - God heard, God remembered, God saw, God knew. (v24-25)
He looks on His people as the son He loves, as the bride He bleeds for. He will use all His might to crush our oppressor, to remove our shackles and lead us to a spacious place. When we hear about the compassion of our redeeming Lord Jesus then we really bow down and worship.