The name of God שדי is written on the back side of the mezuzah scroll. These three letters also mean abbreviation שומר דלת ישראל Guarding the door of Israel. Mezuzah identifies the Jewish house and protects it in this way. Why this identification is needed? Does God not see which house is Jewish, and which is not without any signs? Definitely he does! So what is the purpose of Mezuzah than?
The same question can be asked of commandment to make a sign with blood of passover sacrifice in Egypt. The angel did not hit the Jewish house where this sign was inplace. But again, God definitely knows without any signs which house is Jewish and with is not. Why does He needs these signs to be done?
Kabbalah says the purpose of the creation is freedom of choice. Rav Aryeh Kaplan wrote that the uniqueness of the material world is to put opposite things close to each other and visa versa. The iPhone on my table and iPhone on somebody’s table in New Zealand are far in material world but they are at the same place spiritually because they are the same. The ice cube and lighting candle might be put on the same table but spiritually they are infinitely far. This unique feature of material world is created for our freedom of choice and God wants we choose good as written “…and do good” (Psalms 34:15).
Attaching mezuzah is written Torah commandment (מצווה דאורייתא) “…and write them on doorposts of your house and on your gates” (DEUTERONOMY 6:9) The word מצוה (mitzvah) commandment literally means connection. We are connected in the way we follow somebody’s commandment. Mezuzah is like wifi receiver for our home security. We activate it with a blessing and connect it to the Endless Source of security by attaching it in the proper place.