Before Moses even sets foot in Egypt, God arms him with the ultimate, decisive message that will eventually determine the outcome of the battle against the Egyptian king. This revelation, occurring before the plagues even begin, is primarily meant to fortify Moses against despair. As he faces Pharaoh's relentless stubbornness month after month, knowing the predetermined final blow gives Moses hope and proves that the entire sequence of plagues and their purpose are carefully planned from the start [אור החיים, ביאור יש״ר]. Moses is instructed to hold this ultimate threat in reserve, unleashing it only at the very end when Pharaoh completely severs contact and refuses to see his face again [אור החיים]. The drawn-out process of hardening Pharaoh's heart is not meant to strip away his free will. Rather, God grants Pharaoh the physical endurance to withstand the suffering so that his eventual decision to release the people stems from genuine choice rather than physical collapse [מלבי״ם].
At the heart of this final message is the declaration of the Israelites as God's firstborn son. This title conveys supreme importance, greatness, and profound love, much like how Mount Sinai or the Temple carry God's name [רש״י, רבנו חננאל, רבנו בחיי]. The primary approach among commentators is that while all people and nations are God's children and creations, the Israelites hold the cherished status of the firstborn [בכור שור, חזקוני, ביאור שטיינזלץ, ביאור יש״ר]. This unique standing was earned because they were the first nation to recognize God and choose to serve Him out of love during an era when the rest of the world was lost in idolatry [ספורנו, שד״ל, אבן עזרא]. Furthermore, the concept of a firstborn implies an opening or a release. The Israelites are not God's only children, but rather the firstborn of humanity, the first nation to open the door for the rest of the world, paving the way for a future where all nations will call upon God [רש ר הירש]. In this role, they act as the king's eldest son, assisting God in guiding the spiritual kingdom of the world [העמק דבר].
On a deeper level, God's declaration serves as an official divine endorsement of Jacob acquiring the birthright from Esau [רש״י, הדר זקנים, דעת זקנים]. This birthright rightfully belongs to Jacob because, in the divine plan and the very blueprint of creation, Jacob and Israel preceded everything else [גור אריה, חתם סופר, פרדס יוסף]. Jacob sacrificed greatly to secure this birthright specifically to earn the privilege of serving God and offering sacrifices. By claiming the birthright, he also willingly accepted the heavy burden of exile and enslavement that God had previously promised to Abraham. Therefore, Moses presents Pharaoh with a powerful moral and legal argument. Jacob only accepted the destiny of Egyptian bondage to secure the spiritual right to serve God. By enslaving the Israelites and actively preventing them from worshiping their God, Pharaoh violates the entire purpose of their exile, thereby justifying their immediate liberation [בית הלוי, שפתי חכמים].
Finally, this warning establishes a strict principle of measure for measure. God conveys a clear message to Pharaoh that although He decreed a period of bondage to discipline His people just as a father disciplines a son, Pharaoh maliciously exploited this decree to turn God's firstborn into permanent slaves, cruelly intensifying their suffering [אבן עזרא, רבנו חננאל, רבנו בחיי]. Consequently, if Pharaoh refuses to release God's firstborn, the inevitable punishment will be a direct and devastating strike against his own firstborn [מלבי״ם, קאסוטו]. This profound declaration also reveals that God deeply shares in the suffering of the Israelites. It is only after the final plague, when the Israelites are fully redeemed and walk free, that their true status as the firstborn will be undeniably recognized by the entire world [רבנו בחיי].