In the Old Testament, the relationship between God and Israel was special. There was so much familiarity, so much intimacy, so much grace that it seemed almost paternal in a way.
That was by design. From the outset of the Exodus, God makes His relationship with Israel known. They were His “firstborn.” Full stop.
By this point in their history, the concept of the firstborn was well known to the Israelites. Abraham stressed over having a firstborn of his own – not because he was necessarily obsessed with having children (although he did love the ones he had), but because there was no heir.
The rights of the firstborn extended beyond material possessions, though. As was noted in the life of Jacob-who infamously “bought” the firstborn blessing from his brother Esau-the status of the firstborn extended well through life. It came with a special relationship with God.
But the Israelites weren’t the only ones who understood this “firstborn from a deity” concept. Pharaoh styled himself as the son of a god (the Egyptian god Ra), so when Moses said that Jehovah called Israel His firstborn, in the eyes of Pharaoh, it put the Israelites on equal footing with him. And that’s just simply unacceptable.
This is really the core of the issue in Exodus. Pharaoh doesn’t want to lose his workforce, that much is absolutely sure. But even more than that, Pharaoh doesn’t like being threatened by another God, no matter who It is. It’s a pride issue with Pharaoh, plain and simple.
That’s why God laces the command to Pharaoh with a threat. If Pharaoh doesn’t release the Israelites, God will kill Pharaoh’s firstborn. In short, if Pharaoh threatens God’s firstborn and the lineage that comes with it, God will threaten-and follow through with that threat-Pharaoh’s firstborn and his lineage.
We’ll see in the end who is the true God.