Mezuzot is arguably not biblically required
In my prior essay I offered the idea that it is likely that the command to wear Tefillin is not biblical but was started by Jews who copied the Greek practice to wear amulets. Instead of placing the names of idols in their amulets, the Jews inserted some biblical verses that promised long life. Later, the rabbis spiritualized the practice by calling them Tefillin and stating that they remind Jews to observe the Torah. What about the Mezuzah? Does the biblical phrase “And thou shalt write them upon the door-posts of thy house, and upon thy gates” (Deuteronomy 6:9 and 11:20) require Jews to place Mezuzot on houses? Why was there a disagreement about the contents of the Tefillin boxes in the twelfth century? And what is the meaning of “sign,” what sign/teaching is the Torah emphasizing with the placement on hands, between eyes, door-posts, and gates?
Is the command to place a Mezuzah on the door post biblical?
We saw that Rashbam interpreted the phrase about the hand and between the eyes metaphorically. We also saw that when the rabbis established the law of Tefillin, they did not require that the Tefillin be placed on the hand and between the eyes, but on the arm and forehead. The law about the Mezuzah is similar. While the biblical wording is “write them upon the door-posts of thy house and upon thy gates,” they did not require a writing in these places but the insertion of a parchment into a box or cylinder and the placement to this object on the door-posts of homes. Since the law of Mezuzot follows the metaphoric law of setting a sign on the hand and between the eyes and since the rabbinical law about Mezuzot, like the law of Tefillin, was not enacted precisely like what was written in the Torah, it is reasonable to assume that the statement about writing on the door-posts is also metaphoric.
Were Mezuzot originally protecting amulets as were Tefillin? Probably so. Jews recall that Moses told the Israelites to place blood on their doorposts to protect them from death when the first-born of Egypt were killed in the tenth plague. Moses probably saw this act as a sign of courage and a stimulation to continue to act with valor; for by placing blood on their door-post, the Israelites were openly and bravely declaring that they had sacrificed the lamb, the Egyptian god, to the Israelite God. Yet, despite this meritorious idea, it is likely that many Israelites saw the placement of blood on the door-posts as a magical act to protect them. This is not unusual. It is common human behavior. Many cultures have placed items at the entrance to their homes to guard them from demons. Many early Jews poured salt on the entrance to their homes to assure their safety. Even today, many Jews wear Mezuzot around their neck, as Christians wear crosses, some to announce their religious preference, but others to defend them from harm, even as the cross was used to weaken vampires in the Dracula legends.
Misuse of Mezuzot
Unfortunately, many people identify religious practices and objects with superstition. They feel that objects and practices are holy and have a mystical aura and must be adored, and they feel good about doing it – it gives them a spiritual uplift. They act toward “holy objects” like a husband who worships his wife without helping her, not making her life easy and their lives meaningful.
One example is the widespread custom of kissing holy objects such as the Tzitzit, Tefillin, Mezuzah, and the Torah scroll. The rabbis criticized these well-meaning behaviors when they first began. They are not Jewish in origin and were most likely copied from Muslims and Christians. One should use these items as they were intended to be used rather than showing useless adoration by kissing them. For example, the Tzitzit, Tefillin, and Mezuzah should inspire the study of Torah, and Torah should be studied to acquire some true ideas and to improve oneself and society.
Even the codifier Joseph Caro (1488–1575), who had a mystical bend, criticized the custom of kissing Tzitzit during prayer. Ironically, Caro states that he fears that this practice of kissing Tzitzit might expand and lead to kissing the Tefillin and Mezuzot, which it did.
Why did the rabbis differ about the contents of the Mezuzah as late as the twelfth century?
Probably the answer is that some laws were never really settled or formally canonized till very late. For example, in early Talmudic times, Tefillin were either cylindrical or cubical, but later the cylindrical form became obsolete.
It seems likely that the command to wear Tefillin and place Mezuzot on door-posts is post-biblical and the command in Deuteronomy 6:4-9 and 11:13-21 should be understood as follows:
The verses begin with the command to love God with all your heart (the heart was understood in ancient times as the mind), soul (in the Bible this means the body), and might (all that you have). It goes on to give examples in more detail how and when to remember to love (think about) God. Ten examples are given. (1) God should be constantly on your heart (in your mind), (2) teach it to your children, (3) speak about it at home, (4) and when you leave your home, (5) when you lie down, (6) when you get up, (7) tie them as a sign on your hand (think about God in all your work, (8) and between your eyes (when you think of something and in looking at anything), (9) write them on your door posts (think about God whenever you enter or leave your house), and (10) and on your gates (whenever you enter or leave your city).
 In his Bet Yosef, Orach Chayim 24. He wrote that not kissing was also the opinion of the eighth-century Natronai Gaon and of the ninth-century Moses Gaon. The Gaonim (plural of Gaon) were Babylonian leaders of Jewry from around the late sixth century to 1038.
 Tzvi Adams pointed me to a good discussion on the subject in https://en.wikipedia.org/wiki/Tefillin#Manufacture_and_contents.