The Rosh (Orchos Chaim 25) writes: “One should trust in Hashem with his whole heart, and believe in His individual supervision and concern… believing that His eyes watch the ways of men… One who does not believe in ‘I am the one who took you out of Egypt’ (Shemos 20:2), does not believe ‘I am Hashem your G-d…’ (ibid) This is the uniqueness of Yisrael among the nations, and this is the foundation of the entire Torah.”
The last of the Ten Makos (plagues) was Makas Bechoros — the death of the firstborn. The Jews were told to circumcise themselves (Shemos 12:48) and shecht the Pesach lamb. These bloods would be mingled (Pirke D’rebbi Eliezer) and placed on the sides of the doors and the lintel above the door (Shemos 12:7, 12:22 12:23). Hashem would see the blood and pass by the houses of the Jews when He would smite the firstborn of the Egyptians (Shemos 12:23).
We are told, “I, and not a malach, I, and not an agent, I, Myself, in My Glory.” Hashem Himself would perform the last of the Makos. It would take Hashem Himself to determine if each child was a firstborn or not. The first offspring of the woman is easy to determine, but there is no way to know who is a firstborn of the father. In Egypt they had many extramarital affairs and immoral relationships. One man’s wife may have had many sons who were firstborn to their respective fathers.
In order to remember the great miracle, we are told to perform the mitzva of pidyon haben, to redeem the firstborn son from the Kohein (Shemos 13:12-15). Just as our firstborn sons were redeemed and spared from death in Egypt, so we symbolically redeem our firstborn sons today. However, the mitzva only refers to the firstborn from the mother (ibid). Since the death of the firstborn occurred to the sons of the men also — and the Jewish ones were saved — why doesn’t the mitzva of pidyon haben apply to the firstborn of the men as well? (Rebbi Akiva Eiger)
As we said, to distinguish which sons were the firstborn for the fathers, would require the knowledge of Hashem Himself. However, the firstborn sons of the mothers could be detected by a malach, and this is indeed what happened. Rebbi Akiva Eiger says that what Hashem Himself does is not called a miracle — because Hashem knows and discerns everything all the time. However — what the malach does successfully is a miracle; as the Rabbis say, “Once permission is granted to the destroying angel, he does not distinguish between worthy or unworthy.” The angel is not to be trusted; he may indiscriminately lash out beyond his agency; the Jews themselves were in grave danger! (The destroying angel was allowed into Jewish houses, in case an Egyptian was hiding there.)
Therefore, the fact that the firstborn sons of the women were determined by the malach alone — and none of the Jews were killed — was considered to be a great miracle, and is commemorated by the pidyon haben.
The Blood on the Doorframe
With this, Rebbi Akiva Eiger answered another question: Why was the blood needed on the sides and top frame of the doors? Hashem performed the death of the firstborn, and Hashem can distinguish — without the need for signs.
However, as we have seen, the verses don’t mean that malachim weren’t present. They were indeed present, and fulfilled their assignments properly. This miracle required the merit of the mitzvos of the Pesach offering and the blood of the circumcision.
This was the reason that the Jews were not allowed to go outside the entire night (Shemos, 12:22). As Rashi explains, “Once the destroying angel is given permission, he does not distinguish between worthy or unworthy; at nighttime, permission is granted.” (Rashi, ibid) Outside — they did not have the protection from the mitzva of the blood.
The Obligation to Serve
In the Shema, we describe the Unity of Hashem, accepting the yoke of His Kingship. We accept upon ourselves to serve Hashem, and acknowledge His supervision… See the Ramban at the end of Parshas Bo at great length. Along the way, he explains that one who attaches a mezuza to the door and has proper intent, is acknowledging Creation, Hashem’s knowledge and supervision, belief in prophecy and His mercy. The purpose of all the mitzvos is that we be thankful to Hashem…