The laws of the Torah are divine laws and are beyond the comprehension of the human mind. Any attempt to understand a mitzvah or the reasons for a mitzvah will fall short of complete comprehension. Indeed, there are many laws that defy any sort of explanation. One cannot, therefore, make his understanding of a mitzvah or any of its laws, a precondition to its performance. This does not prohibit us from delving into and seeking to understand possible reasons for mitzvahs. Quite the contrary, we are duty bound to seek understanding of mitzvahs as this increases our appreciation of them.
Our understanding of the mitzvah of mezuzah must begin with a discussion of the sentences contained within Shema Yisroel (Hear O Israel), Deut. 5:4-9 and Vehaya Im Shamoa (And it will happen if you obey), Deut. 11: 12-21.
The first paragraph of the mezuzah contains the most fundamental concept of Judaism, the unity of G-d. This concept, that all the diverse phenomena that exist in this world are the result of One source and not the result of conflicting forces, is the foundation of our faith and is what sets Judaism apart from all the ancient and modern polytheistic ideas and religions.
The second paragraph of the mezuzah teaches the principle of divine reward and punishment. The mezuzah is a reminder in times of prosperity that our abundance is dependant on the observance of the Torah and in times of deprivation that misfortunes come as a punishment for our sins.
With an appreciation of the importance and the sacredness of the content of the mezuzah we begin to realize that the laws that require such exactness and sanctity in the writing of the mezuzah are a reflection of the sanctity of its content. Moreover, one can readily see how the stringency of the laws pertaining to mezuzah impress upon a person exactly how important a document this must be to require such exactness.
A mezuzah is not an amulet and Maimonadies indicates that to treat it as one degrades the mitzvah. Nevertheless, one of the benefits of the mitzvah of mezuzah, as well as any mitzvah, is that it protects he who performs the mitzvah from harm that may otherwise befall him.
The Talmud relates that when Onkeles, adopted the Jewish faith the Roman Emperor sent a company of soldiers to arrest him. As he was taken from his home, he stopped to place his hand over the mezuzah. When the soldiers asked him to explain the meaning of his action he replied, "A mortal king lives within and hires guards to stand outside to protect him but the Almighty has his servants inside while he stands outside to protect them as scriptures says, "G-d shall protect you as you come and as you leave, forever'". The soldiers were so impressed by this remark that they let him go and they, too, converted.
Maimonadies concludes the laws of mezuzah with this statement: One must be very punctilious in fulfilling this mitzvah because it is a constant obligation that includes everyone. Every time a person enters and leaves he is confronted with the unity of G-d, the name of the Almighty and he remembers his love for him. He awakens from his slumber and his involvement in the vanity of temporal matters and realizes that nothing has eternal existence but the knowledge of G-d. He then immediately returns to the path of righteousness. Our sages have said that he who has tefillin on his arm and head, tzitzis on his garment and mezuzahs on his doorways can be assured that he will not sin because he has many reminders. These are the selfsame angels that protect him from sin as it says, "The angels of G-d surround those who fear him and they protect him".