Rabbi Roodyn

Hashem did not seek to build a nation of free individuals, Klal Yisrael is to be built on households, formed by blood ties or free choice. Only within the framework of the family, formed by the will of its members, can the individual find gratification in his own personality. This is symbolised by putting the blood on the mezuzos and the lintel of the house in which the lamb is to be consumed by the same  people who offered it.

The mezuzos and lintel represent the entire represent the entire house. The home is comprised of two elements, social insulation vis-à-vis society and physical insulation against the forces of nature. Social insulation is signified by the doorposts, the word mezuzah being related to the word zuz to move, namely to control the flow of those who enter and exit. The lintel in Hebrew is a mashkof, related to the world lehashkif, to look down from above. This represents the root which provides protection against the forces of nature.

When the slave becomes a man, a father and a son, in the family of his own, he receives legal and physical protection from Hashem as represented by the mezuzos and mashkof. First, however he must completely devote himself to Hashem, surrendering to Him his whole former essence, as represented by the slaughtering of the Pesach. Only then can he find gratification in his own personality as represented by the eating of the Pesach.

This is symbolised by the blood of the lamb on the doorposts and the lintel of the house in which the is to be consumed. It is not the walls that protect the person from the hostile human elements nor is it the roof that protects him from the hostile elements of nature. Rather it is Hashem who between the walls and under the roof, protects those who devote themselves to Him.