“I will harden his heart” – in order to administer their sentence. “You shall say to Pharaoh: So said the Lord: My son, My firstborn, Israel” (Exodus 4:22). “You shall say to Pharaoh: So said the Lord: My son…” – the Holy One blessed be He revealed to him that Pharaoh will not let Israel go until the plague of the firstborn; therefore, it was not necessary for the verse to inform him of it later.8When God said to Moses: “One more blow I will bring upon Pharaoh and upon Egypt; afterward he will send you forth from here” (Exodus 11:1), He did not specify what that blow would be.
However, Moses knew that if it was the last blow it must be the plague of the firstborn, and he warned Pharaoh accordingly (Exodus 11:5). What is “My son, My firstborn, Israel”? It is a reference to Jacob their ancestor, who purchased the birthright from Esau so he would be able to serve God.9The firstborn were initially designated to perform sacrificial rites in service of God. Israel is another name for Jacob.
You shall say to him: “Let My son go, and he will serve Me” (Exodus 4:23). He said to him: ‘If you prevent My firstborn from serving Me, I will prevent your firstborn from serving you, as I will kill them all.’