There is a widespread notion that a mezuzah - which it is a mitzvah to fix to the right doorpost of every room in your house except the bathroom - is a sort of Jewish good luck charm, a source of protection for the home. The passage from the second paragraph of the Shema that speaks of mezuzah implies as much, saying "in order that your days shall be long, and the days of your children". The letters shin, dalet, yud, spelling Shaddai, one of God's names, appears on the outside of most mezuzot. Kabbalistic sources point out that these letters also stand for Shomer Daletot Yisrael, "who guards the doors of Israel".
Maimonides, however, strongly objected to the practice of those in his time who would add additional names of God and angels to the mezuzah text for extra protection. He wrote of them; "These fools not only nullify the mitzvah, but furthermore, they make from a great mitzvah ([that reflects] the unity of the name of the Holy One, blessed be He, the love of Him, and the service of Him, a talisman for their own benefit". For Maimonides, the mezuzah was a constant reminder to awaken from our habitual sleepwalking through life and remember what really matters whenever we enter a home.