Locks on doors? Check.
Alarm System? Check.
But hold on, are you sure? You see, many people are aware that Mezuzahs protect your home (well, more accurately God protects your home in the merit of the Mezuzah). But you may not know that if you only have one Mezuzah on your front door, you're simply not covered!
Here's how it works. A Jewish home is obligated to have a Mezuzah on almost every door. There are a few exceptions like bathrooms, small closets etc. (You can read more about it here). When all of the necessary doors have Mezuzahs, they form a spiritual barrier against negative forces that enter the home. But if one or more Mezuzahs in missing, or not kosher, there is a hole in that barrier.
On the outside of every Mezuzah scroll you will find written, "Sha-dai", one of God's holy names. There are many names for God in the Torah. The Kabbalists tell us that this particular name was chosen to be displayed on the outside of the Mezuzah because of it's special protective powers. For example, the three Hebrew letters, "Shin", "Dalet" and "Yud" which form the name "Sha-dai" are actually an acronym for the Hebrew words, "Shomer Daltot Yisrael" which means "Guardian of the doors of Israel"!
Its also fascinating that if you remove the last letter of this name of God, you get a Hebrew word with a very different meaning. The letters "Shin" and "Dalet" form the word "Sheid" which is usually translated as "demon", but really means "negative spiritual forces". The Kabbalists say that having this particular name of God (Sha-dai) on the outside of the Mezuzah, keeps away all negative forces (Sheid).
Imagine if you had a security system installed in your home but only on the front door. Would your home be secure? It may be better than nothing, but certainly its far from being completely secure. Mezuzahs work in much the same way. For them to do their job properly, they need to be on every door in the home which requires one.
Try it and you'll see! When you have Kosher Mezuzahs on every door of your home, you will feel something different in your home. Just ask anyone who has tried it and they will tell you. And guess what? It will cost a lot less than having a security system installed!