What does Exodus 12:12 mean?
When God first commissioned Moses to lead Israel out of slavery, He predicted terrible judgments on Egypt (Exodus 3:19–20). He also warned Moses that it would take the direst threats to make Pharaoh finally give up the captive Hebrews (Exodus 4:21–23). After nine catastrophes (Exodus 7:21; 8:6, 17, 24; 9:6, 10, 24; 10:14, 23), the Lord is about to make good on His promise of a tenth plague (Exodus 11:4–6). This will involve the death of every firstborn—the primary heirs—from animals to servants to the household of pharaoh himself. This will be the ultimate proof that the God of Israel, not the idols of Egypt, have divine power.Instructions in this chapter are for the Israelites to avoid being caught in this judgment. By marking their homes with lamb's blood (Exodus 12:3–7), they demonstrate faith and obedience to the Lord. Over such homes, He will "pass over" in His wrath (Exodus 12:13). That will reinforce the idea that God favors the people of Israel over their oppressors (Exodus 11:7).
When the judgment comes, it will so devastate Egypt that they will demand the Israelites leave immediately (Exodus 12:29–33).