One question about the mezuzah is what words are to be written down (6:4-11:20?) as the Torah does not specify what these words are. A second question is how were the words to be written down? Tigay (1996, p.79, 443, 444) notes that "the text implies that the words were to be inscribed directly on the doorposts and gate structures." This seems to have been the Samaritan's understanding of the law as to this day by their homes on Mount Gerizim their mezuzot are either inscribed on the wall or on a plaque which is attached to the wall. (I thank Ari Zivotofsky for sending me pictures of these mezuzot.) Derby (1999) further suggests that the phrase "on your gates" means that the words were to be written on a "stela in a plaza within the gates." However, Chazal interpreted 6:9 and 11:20 to mean that the verses 6:4-9 and 11:13-20 are to be written on a parchment and that the parchment is placed in a case which is attached to the doorpost. (The term mezuzah is technically the doorpost, but traditionally it has been understood to refer to the case with the parchment.)
It seems that the point of the mitzvah is for a person to be reminded of the words that were written down when the person passed the door or the gates, see Rambam, Laws of mezuzah, 6:13. This could have occurred if the words were written directly on the doorposts or if the words were hanging on the doorposts since then a person would see the words when he/ she would pass by. However, even if the words are enclosed within a case, a person can still recall the words when he/ she sees the mezuzah.
However, over the years people began to see the mezuzah as a type of amulet to protect the house and the people associated with the house. Some passages in the Talmud (Menachot 33b, Menachot 43b, Avodah Zara 11a, Yerushalmi Peah 1:1) would seem to indicate this idea, but Gordon (1977) disputes this understanding of the passages. He argues that Chazal did not view the mezuzah as having any protective role. One of his proofs is that an arched doorway does not require a mezuzah, but if the mezuzah was to protect the house, then this protection should be needed even if the doorway was arched. Thus, maybe one can say that in Talmudic time, not all of Chazal viewed the mezuzah as a form of protection for the house and people, but for sure many did.
In the period after the Talmud, it seems that almost everybody believed that the mezuzah was to protect the home and its inhabitants. The name of G-d, Shad-dai, was added to the back of the parchment and it was decreed that the mezuzah could only be written on certain propitious times (Trachtenberg, 2004, pp. 147, 148, see also Y. Liechtenstein, 1989, p. 419). More importantly, the names of angels, figures, which were believed to have magical powers, and various verses, were added to the parchment. This occurred both by the Sefardim, as can be seen by mezuzot that were found in the Geniza (see Bohak, 2009-2010) and by Ashkenazim (see Sperber, 2007, pp. 96-124). Perhaps the most famous example is from the Ravyah (Germany 1140-1225, see Y. Liechtenstein, p. 421 or Sperber, p. 123) where the names of angels and figures can be seen within the verses of Devarim 6:7, 6:8, there are figures at the end of various verses and names of angels on the side. Also, at the bottom of the mezuzah there are several verses from Tehillim. All of these additions were to enhance the protective powers of the mezuzah. (One might wonder why there was a need for extra protection but I guess the idea was that these elements could only help more.)
Almost all of these additions came to an end with the Rambam (1135-1204) who seems to have been one of the few people who objected to this practice. He writes (Laws of mezuzah 5:4, translation from Twersky, 1972, pp. 94,95) "It is a universal custom to write Shad-dai (Almighty) on the other side of the mezuzah, opposite the blank space between the two sections. As this word is written on the outside, the practice is unobjectionable. They, however, who write names of angels, holy names, a biblical text, or inscriptions usual on seals within the mezuzah are among those who have no portion in the world to come. For those fools not only fail to fulfill the commandment but they treat an important precept that expresses the Unity of G-d, the love of Him and His worship, as if it were an amulet to promote their own personal interests; for according to their foolish minds, the mezuzah is something that will secure for them advantage in the vanities of the world."
After the Rambam's harsh words, which scribe would want to lose his place in the next world or be called a fool? Hence, this practice of adding names of angels, verses and figures ended, see comments of Haghot Maimonides on Rambam, Laws of mezuzah 5:4. However, the belief that the mezuzah was a form of protection continued. Thus, the Tur (1275-1340, Yoreh De'ah 285) writes at length how the mezuzah gives one long life and protects one's house, though he concludes very briefly that these benefits should not be the reason why a person fulfills the commandment.
One practice that began (continued?) is that the words "G-d, our G-d, G-d" from Devarim 6:4 is written in code (by writing the following letter for each of the real letters) on the outside of the mezuzah, see comments of Haghot Maimonides on Rambam, Laws of mezuzah 5:4. The Tur (end of 288) writes that this was the custom in France and Germany. It seems the Rambam did not know of this custom since he did not mention it, and this practice continued since the words are written on the outside of the parchment just like Shad-dai.
Another new custom was to touch the mezuzah. The Darkei Moshe (R. Moshe Isserles, 16th century, on the Tur 285) quotes that the Maharil (Germany 1365-1427) said that when a person leaves his house to go on a journey he should touch the mezuzah and recite the words "be-shimkha tal atleh" (from a piyyut) since the numeric value of the letters (gematria) of the word "tal" equals the value of the letters of the word G-d that is written in code on the back side of the parchment. The Maharil also said that when a person leaves his house, he should touch the mezuzah and recite the verses from Tehillim 121:4,7, which were two verses on the bottom of the mezuzah of the Ravyah. This practice continues the idea that the mezuzah protects, but now it was not the house or a person within the house who sought protection but one who left the house.
The Darkei Moshe notes that this idea of touching the mezuzah can be found in the Talmud (Avodah Zara 11a), but if this was really the source for this custom it is odd that nobody mentioned touching the mezuzah until the Maharil. More likely, this custom developed in the end of the Middle Ages, either as a popular practice or the Maharil suggested it. With either possibility it is not clear why it suddenly developed, but maybe it was a re-action to the Rambam's ruling against adding the names of angels, figures and verses to mezuzot. The idea being is that if the people could not add names of angels, figures, and verses to the mezuzah, then instead they would recite verses on the mezuzah (the same verses that had been added to the mezuzah) and they touched the mezuzah to connect these verses with the mezuzah.
The Shulchan Arukh does not mention touching or kissing the mezuzah, but Rama in his comments on the Shulchan Arukh (285:2) quotes the Maharil again but with two small changes. One, he does not mention the case of going on a journey. Two, he adds that when one enters a house one touches the mezuzah, but he does not state that a person recites any verses in this case. This addition would be the idea that the mezuzah protects a person when entering the house, but it is not clear why no accompanying verse was recited. It could be that this was the practice that had developed from the time of the Maharil to the Rama. We see that up to the middle of the 16th century, the practice amongst some Ashkenazim was to touch the mezuzah by the opening of the house, but there is no record that anybody kissed the mezuzah.
This touching of the mezuzah would naturally lead one to kiss the mezuzah, and the kissing continues the idea of viewing the mezuzah as an amulet since the kiss would be for good luck. The custom became popular amongst the kabbalists who claim that the Ari instituted the custom of kissing the mezuzah and Hasidim who follow the kabbalists. It also spread to the Mitnagdim, as R. Avraham Danzig (1748-1820, Chayyei Adam 15:1) writes that when a person leaves his house, he should kiss the mezuzah. It is interesting that he does not mention that one should recite any verses when kissing the mezuzah and he only refers to kissing when one leaves the house. (Again was R. Danzig recording the custom of his time or was he writing what was his belief as to what one should do?) I think today the people who kiss mezuzot kiss them by every room and not just when leaving their house.
Would the Rambam have approved of this custom of kissing the mezuzah? The Rambam did not address this question since in his time this custom did not exist, and hence we have to infer what he would have thought. My guess is that his opinion would depend on why a person is kissing the mezuzah. If a person kisses a mezuzah because he thinks it will protect him from danger, as for example if one makes a special effort to kiss the mezuzah before he/ she goes to sleep, then most likely the Rambam would be upset just as he was when people added names of angels, figures and verses to the mezuzah since the person is subverting the law to his own personal benefits. (Thus, I have been told by people not to kiss the mezuzah.) However, if a person kisses the mezuzah to show his love of the law or of the Torah, which seems to be R. Danzig's understanding of the custom, then maybe the Rambam would have approved.
In conclusion, the idea of the law of mezuzah in the Torah seems to have been to encourage people to contemplate the verses contained in the mezuzah, but over time there developed the idea that the mezuzah is to protect people as a type of amulet. The Rambam tried to end this practice but it has continued in the popular belief of people. It is likely that this belief is the reason for why the custom developed since the 17th century to kiss the mezuzah, while others view the kissing of the mezuzah as expression of their devotion to G-d. Hence we have a practice, which some people view as extremely praiseworthy and indicates of high level of religiosity, while others, admittedly a much smaller group, view negatively.
Bohak, Gideon, 2009-2010, Mezuzot with magical additions in the Cairo genizah, Dine Israel, 26-27, pp. 387-403.
Gordon, Martin, L., 1977, Mezuzah: Protective amulet or religious symbol, Tradition: A Journal of Orthodox Jewish Thought, 16:4, pp. 7-40.
Lichtenstein, Yechezkal, 1989, The mezuzah as a protection for the house, Techumin, 10, pp. 417-426.
Sperber, Daniel, 2007 (vol. 8), Minhagei Yisrael: Sources and history, Jerusalem: Mossad Harav Kook.
Tigay, Jeffrey H. 1996, The JPS Torah Commentary: Deuteronomy, Philadelphia: The Jewish Publication Society.
Trachtenberg, Joshua (1904-1959), 2004 (re-print of 1939 edition), Jewish magic and superstition: A study in folk religion, Jerusalem: Sefer ve-Sefed Publishing.
Gordon, Martin, L., 1977, Mezuzah: Protective amulet or religious symbol, Tradition: A Journal of Orthodox Jewish Thought, 16:4, pp. 7-40.
Lichtenstein, Yechezkal, 1989, The mezuzah as a protection for the house, Techumin, 10, pp. 417-426.
Sperber, Daniel, 2007 (vol. 8), Minhagei Yisrael: Sources and history, Jerusalem: Mossad Harav Kook.
Tigay, Jeffrey H. 1996, The JPS Torah Commentary: Deuteronomy, Philadelphia: The Jewish Publication Society.
Trachtenberg, Joshua (1904-1959), 2004 (re-print of 1939 edition), Jewish magic and superstition: A study in folk religion, Jerusalem: Sefer ve-Sefed Publishing.
Twersky, Isadore, 1972, A Maimonides Reader, New York: Behrman House.