When God said "And I shall see the blood" regarding the Passover in Egypt, the Mekhilta offers a stunning alternative reading. The "blood" God would see was not the blood of the Passover lamb at all. It was the blood of the Binding of Isaac.
The connection runs through a verbal link. (Genesis 22:14) records that Abraham "called the name of the place 'The Lord will see.'" The word "see" in both contexts — God seeing the blood in Egypt, and the place where God "will see" — creates a bridge between two foundational events in Jewish history. The merit of Isaac's near-sacrifice, performed generations earlier, was what God "saw" when He passed over the Israelite homes.
The Mekhilta brings further support from (1 Chronicles 21:15): "But as He was about to destroy, the Lord saw and He repented Himself of the evil." What did God see at that moment of impending destruction? Again, the blood of the Binding of Isaac. The merit of that ancient act of devotion continued to shield Israel across centuries.
This teaching transforms the Passover from a single historical event into part of a continuous chain of merit. The Israelites in Egypt were not saved solely by their own actions — painting lamb's blood on their doorposts. They were saved because God looked at those doorposts and remembered a father who was willing to offer his son, and a son who was willing to be offered. The blood of the lamb triggered the memory of an even deeper sacrifice.