The Mitzvah of Mezuzah is actually commanded in Devarim 6:9. But in Shemot 12:3-15 the reason for the Mitzvah is explained: "Speak to the entire assembly of Israel, saying: On the tenth of this month each person shall take for themselves a lamb or a kid, for each household...[on] the fourteenth of this month, the entire congregation of the assembly of Israel shall slaughter it in the afternoon. They shall take some of its blood and place it on the two doorposts and on the lintel of the houses in which they will eat...I shall go through Egypt on this night, and I shall strike every firstborn in the land of Egypt, from man to beast; and against all gods of Egypt I shall mete out punishment. The blood shall be a SIGN for you upon the house where you are; when I see the blood I shall PASSOVER you; there shall not be a plague of destruction upon you when I strike the land of Egypt. This day shall be a remembrance for you and you shall celebrate it..."
An Oht (a sign) is significant because it is a reminder of an act that occurred before the eyes of the entire nation.
The mezuzah (מזוזה), representing the protective blood sign, is spelled with two letters zayin. The two letters zayin united together (זוז) within one word for the Jewish ritual item upon the doorpost is like "one dagger of double edges (זוז) of her (מה)." In other words, the "blood sign" on the doorpost which protects a Jew's home in mitzrayim is a woman's double-edged dagger. The contemplative power to "connect and interrelate all elements within creation", bringing down Divine Will into Understanding, is a function of the letter vav which forms the spine of the dagger of double-edges.
The witch's double-edged ritual dagger is called an athame (pronounced a-tham-ay). Silver Ravenwolf writes regarding the Celtic history of the athame :
We know from Celtic history that, for a time, their weaponry through use of iron was far superior to their contemporaries, and that they were feared because of it. We also know that such folklore practices as hanging a knife above a door to cut any negativity that might enter the home (which means that the knives were not used to physically cut, but to defend on the astral plane) were highly popular in that culture.
A witch's athame is never ever used to draw blood. If it ever does, it becomes ritually unfit and can never be used again. It must be buried. Thus, in ancient Celtic culture the protecting blood sign upon the door was a double edged dagger which never draws blood. In other words, like the mezuzah, it is a protective blood sign which never draws blood.
Putting these ideas together, we can see that the Jewish ritual mezuzah and the Celtic ritual athame both refer to the same essence of spiritual protection - to guard the house from all negative influences that might attempt to enter it. Even the construct of the word mezuzah contains within it the idea of a woman's double-edged dagger as the protective device.
 Solitary Witchcraft, Silver Ravenwolf (p. 125)