The Angel of Death is coming to town. How do you let him know that you are one of the good guys?
In the case of the Jewish people on that last fateful night in Egypt, this was not accomplished by waving a flag, or by saying tehillim. Instead Moses instructs the Jewish people to do a very specific act:
…kill the Passover lamb. And you shall take a bunch of hyssop, and dip it in the blood that is in the basin, and strike the lintel and the two side posts with the blood that is in the basin; and none of you shall go out from the door of his house until the morning. For the Lord will pass through to strike the Egyptians; and when he sees the blood upon the lintel, and on the two side posts, the Lord will pass over the door, and will not let the destroyer come into your houses to strike you. Ex 12:21-23
What does this mitzvah have to do with anything? How are the combination of hyssop, blood, and doorposts the symbol of the Jewish people?
The answer is that this particular commandment embodies our very essence!
Remember that the overarching mission of the Jewish people is to take elements from the physical world, those things made by G-d, and to elevate them to the spiritual plane. So in taking hyssop (which is a low grass), and dipping it in blood which is then smeared on the doorpost, the Jews were literally combining a living item from the plant world, and one from the animal kingdom, and then moving them up, to the doorpost and lintel. We elevated G-d’s creations to the spiritual plane – the height of the human head, where our soul resides.
The symbolism of the doorpost helps us understand this commandment even further. G-d respects the creations of mankind. In all of the plagues of Egypt, in none of them are the buildings harmed. G-d seems to show an inherent deference to human ingenuity and creativity, the things that we build. And human creation is not meant to stand alone: as this mitzvah tells us, we are supposed to elevate G-d’s creations by combining them with our own. We are meant to use technology as the vehicle for the elevated physical materials.
And of course a doorpost also represents the home, the relationship between husband and wife that mirrors our personal and national relationship with G-d. When we choose to protect our homes by publicly identifying as the people who know their role in this world, then we have identified ourselves as G-d’s people.
The commandment of using the hyssop and blood was only in force that one evening, but it is connected to the commandment of the mezuzah – the scroll containing the words of the shema that are also supposed to go on the doorpost. The scroll of the mezuzah is made of animal parchment, combined with vegetable ink – and then placed on our doorposts. The mezuzah is an exact parallel of that first doorpost commandment, reminding us and Hashem that we understand our purpose on this world, and are reminded of it every time we come and go.