Moses told the Israelites to take a lamb for the Passover offering, and they were terrified. The Mekhilta preserves their fearful protest: "Will we slaughter the abomination of Egypt before their eyes and they not kill us?" (Exodus 8:22). The lamb was sacred to the Egyptians. Killing it publicly would be like setting fire to a temple. The Israelites expected a massacre.
Moses did not dismiss their fear. Instead, he reframed it. The Torah says the lamb must be kept "for a keeping" — meaning the Israelites had to select the lamb and tie it up for four days before slaughtering it (Exodus 12:21). Moses pointed out that this extended waiting period was itself a test and a sign. If God could protect them during the most provocative part — openly selecting and binding Egyptian sacred animals in full view of their neighbors — then surely He would protect them during the slaughter.
The logic was almost daring. The "keeping" was not just about inspecting the lamb for blemishes, as later halakhah (Jewish religious law) would emphasize. It was a four-day demonstration of divine power. Every day that passed with the lamb tied to an Israelite bedpost and no Egyptian mob forming at the door was proof that God was already intervening. By the time the fourteenth of Nissan arrived and the moment of slaughter came, the Israelites had already witnessed the miracle. They had lived inside it for four days.
The Mekhilta is making a profound point about faith: sometimes God does not remove the danger. He walks you through it slowly enough that you can see His hand at work.