The second paragraph of the Shema, which appears towards the end of the parasha of Ekev, describes the dangerous consequences of turning aside from following Hashem and his Torah and becoming enmeshed in idolatry. What happens next is:
וְחָרָה אַף ה׳ בָּכֶם וְעָצַר אֶת הַשָּׁמַיִם וְלֹא יִהְיֶה מָטָר וְהָאֲדָמָה לֹא תִתֵּן אֶת יְבוּלָהּ וַאֲבַדְתֶּם מְהֵרָה מֵעַל הָאָרֶץ הַטֹּבָה אֲשֶׁר ה׳ נֹתֵן לָכֶם.
Hashem's anger will then be directed against you, and He will lock up the skies so that there will not be any rain. The land will not give forth its crops, and you will rapidly vanish from the good land that Hashem is giving you. (Devarim 11:17)
But immediately after this prophecy of dispersal, come the mitzvot of wearing tefillin, of learning and teaching Torah, and of mezuza.
וְשַׂמְתֶּם אֶת דְּבָרַי אֵלֶּה עַל לְבַבְכֶם וְעַל נַפְשְׁכֶם וּקְשַׁרְתֶּם אֹתָם לְאוֹת עַל יֶדְכֶם וְהָיוּ לְטוֹטָפֹת בֵּין עֵינֵיכֶם.
וְלִמַּדְתֶּם אֹתָם אֶת בְּנֵיכֶם לְדַבֵּר בָּם בְּשִׁבְתְּךָ בְּבֵיתֶךָ וּבְלֶכְתְּךָ בַדֶּרֶךְ וּבְשׇׁכְבְּךָ וּבְקוּמֶךָ.
וּכְתַבְתָּם עַל מְזוּזוֹת בֵּיתֶךָ וּבִשְׁעָרֶיךָ.
Place these words of mine on your heart and soul. Bind them as a sign on your arm, and let them be an insignia in the center of your head.
Teach your children to speak of them, when you are at home, when traveling on the road, when you lie down and when you get up.
[Also] write them on [parchments affixed to] the doorposts of your houses and gates. (Devarim 11:18-20)
One may ask why these mitzvot, which have each been presented earlier in the Torah, are repeated. But in particular, why do they appear in the context of the loss of the land and subsequent exile?
Rav Shimson Rafael Hirsch (19th century, Germany) sees the significance of these particular mitzvot that are to be performed in exile in the following way:
Even if you are exiled from your land, and no longer have your own soil to subordinate to God’s Torah, you must take God’s Torah with you wherever you may go. Your hearts and your souls, the thinking, aspiring, and achieving aspects of your personalities, remain committed to God’s Torah. By placing the tefillin-sign on your arms and heads, you constantly remind yourselves and others of this commitment to the Torah. No matter where you are, you remain bearers of God’s Word, you remain the people of God's Torah, and you are duty-bound to educate your children in this spirit and to devote all your own lives, at home and outside the home, to this purpose.
This echoes a comment of Rav Hirsch (Shemot 25:17) regarding why the poles inserts into the sides of the Ark containing the luchot could never be removed:
The poles, the means of carrying the Ark, symbolically represent the command and the mission to carry the Ark and its contents, if it becomes necessary, away from the precincts of its present position. The command that these means of transport never be lacking is to emphasise in our minds the fact that from the very beginning it must be made clear that this Torah and its transmission is in no way bound or confined to the place or existence at any time of the Temple and Sanctuary.
Exile is no bar to connection with Torah. On the contrary, Jews who have been sent away from the land of Israel can, and must, re-double that connection in their new environment.
Rashi (11th century France) expounds on the juxtaposition of exile and these mitzvot by saying:
אף לאחר שתגלו הוו מצויינין במצוות, הניחו תפילין, עשו מזוזות, כדי שלא יהו עליכם חדשים כשתחזרו.
Even after you have been banished be marked by mitzvot: put on tefillin, make mezuzot, so that they will not be new to you.
Unlike his Midrashic source, which exhorts that mitzvot in general should not be new when we return to Israel, Rashi specifies the mitzvot in the original text (i.e. in Devarim 11:18-20). But this raises the obvious question: Tefillin and mezuzah are not mitzvot dependent on the land of Israel (unlike, for instance, agricultural laws such as terumah and ma’aser). Why would we have imagined that they would not apply outside of Israel, and need a verse to tell us otherwise?
This question leads the Ktav VeKabalah (19th century Germany) to suggest that maybe our text of Rashi is corrupted. Perhaps Rashi originally referred to teruma and ma’aser as mitzvot that might have supposed to not apply outside of Israel (whereas in fact there is a rabbinic obligation to separate tithes in the parts of Mesopotamia, that were incorporated into the Davidic empire). A scribe in a later age then abbreviated תרומה ומעשר as תו״מ, which another scribe, at an even later time, expanded into תפילין ומזוזה. Nevertheless this ingenious theory is undermined by versions of Rashi copied by early sources such as the Ramban which include תפילין ומזוזה.
The Mizrachi (16th century Turkey) suggests that in exile, living in borrowed or rented homes, we might think that are exempt from the mitzvah of mezuzah. And whilst suffering the trials and tribulations of being in a foreign land, we would not have the requisite concentration that must accompany the wearing of tefillin.
Rashi’s source, the Sifrei (Ekev 43) (2nd century, Israel), brings a parable:
אף על פי שאני מגלה אתכם מן הארץ לחוץ לארץ, היו מצוינים במצוות, שכשאתם חוזרים לא יהיו לכם חדשים. משל למלך שכעס על אשתו וחזרה לבית אביה, אמר לה: הוי מקושטת בתכשיטיך, כשתחזרי לא יהיו עליך חדשים. כך אמר להם הקב"ה לישראל: בני, היו מצוינים במצוות, שכשאתם חוזרים לא יהיו עליכם חדשים.
Even though I am exiling you from the land to the Diaspora, be marked by the mitzvot so that when you return they will not be as new to you. A parable: a king who became angry at his wife and she returned to her father’s house. He said to her: “Adorn yourself with your jewelry so that when you return they will not be new to you.” So said Hashem to Israel: My children, be marked by mitzvot so that when you return they will not be new to you.
The Ramban (13th century Israel) says that this midrash conveys a “profound secret” that he has “already alluded to” (in Vayikra 18:25). Although it is not clear what the Ramban means by his “allusion”, it is likely that he is referring to his premise that Hashem’s relationship with the land of Israel is unlike His connection to anywhere else. Every other land is administered by some heavenly body on behalf of Hashem, but Israel is overseen directly by Hashem Himself.
That would explain why there is even a supposition that mitzvot would not apply outside of Israel and outside of Hashem’s direct involvement, as suggested by the Midrash.
The Maharal (16th century Prague) adds, in his effort to “reveal the secret” of the Ramban, that mitzvot are called “ משפט אלהי הארץ” - “statutes of the G-d of the land” - in Melachim Bet 17:26,27, consolidating the notion that mitzvot belong in Israel but not elsewhere. Even though the midrash refutes this idea, we are left with the understanding that Israel is the natural place for mitzvot in a way that is not replicated outside.
The Ktav VKabala also believes that there is a “profound secret” in the Midrash’s parable, though of a more rationalist nature. The king’s wife needs to wear her adornments, even when banished to her father’s house, to show that she has not given up hope of returning to her husband. Thus the wearing of the adornment demonstrates the extent of her continual link to her husband the king and to his palace, and her love and desire to return.
The parable therefore teaches more than just that mitzvot in exile are “for practice” as a superficial understanding might suggest. Tefillin, mezuza and indeed every mitzvah that we perform while outside of Israel are an expression of our ceaseless link with the “king’s palace” - the land of Israel itself - and with “the king” - Hashem.