And Hashem has declared you this day to be a people for his own possession, as He has promised you, and that you should keep all his commandments;
This possuk is teaching us another aspect of our connection to Hashem. Even if another nation were to improve their ways and draw close to Hashem as much as possible, they will never reach the level of Klal Yisroel.
Klal Yisroel is Hashem’s treasured possession, and that is something that will never change.
Hashem promised us that we will be His treasured nation. When He invited us to accept the Torah, He told us that we would be His treasured among all the nations.
That is the meaning of this possuk. Hashem has chosen us as his treasured possession, and elevated us to the level of His nation, as He promised us when He gave us the Torah.
Another explanation the Ohr Hachaim offers is that even if ch”v Klal Yisroel angers Hashem, He will never exchange us for another nation.
Hashem promised us this when we made the Golden Calf. Moshe Rabbeinu then davened to Hashem and asked Him that Klal Yisroel should be outstanding from all the nations in the world – ונפלינו אני ועמך מכל העם אשר על פני האדמה. The Gemara explains that Moshe was davening that Hashem not rest His Shechina on the world's other nations.
In his Sefer Chefetz Hashem on Shas, the Ohr Hachaim explains that even when the other nations act in an exemplary fashion, and Klal Yisroel is seriously deficient in its Mitzvos, Hashem would still only rest His Shechina on His own nation. Hashem agreed to this Tefilla; since then, the only place the Shechina is to be found is with Klal Yisroel.
The Torah continues that Hashem has elevated us as His nation ולשמור כל מצותיו – to guard all of His Mitzvos.
This does not seem to make sense. When Klal Yisroel chooses Hashem, it would be appropriate to say that Klal Yisroel accepted upon themselves to guard His Mitzvos. However, in this possuk, we are discussing that which Hashem did for Klal Yisroel by choosing them as His treasured nation. Why mention the guarding of the Mitzvos in this possuk?
The explanation could be, explains the Ohr Hachaim, that the beauty of Klal Yisroel that Hashem granted them is that He elevated them with the giving of his Mitzvos. He did not give those Mitzvos to those that preceded them. Adam, Noach, Avraham, Yitzchok, and Yaakov, all served Hashem, yet they were not granted the opportunity to do Mitzvos. Avraham, Yitzchok, and Yaakov were given some Mitzvos to keep, but that was only a tiny sampling of the Torah. Klal Yisroel were elevated and purified to be able to accept these Mitzvos, and they are privileged to fulfill them.
Mitzvos, in general, are tools to clean a person’s soul and psyche from the dross that accumulates on them from the impurities of this world. By granting us these Mitzvos, Hashem had granted us the opportunity to clean our souls and purify our psyches.
Additionally, sometimes these Mitzvos can be quite difficult for a person to fulfill. When Hashem announced that He had chosen Klal Yisroel to fulfill His Mitzvos, He announced that He would give them opportunities to perform them.
This is a special privilege Hashem gave His nation, and we are thankful for it.
Another explanation is based on the words of the pessukim and the Gemara. The possuk writes, ‘one who guards a Mitzvah will not know any evil.’ The Gemara deduces that while a person is doing a Mitzvah, nothing untoward will happen to him, and no damage can occur to him.
The Torah is telling us that Hashem will guard us with all of these Mitzvos, that while we are doing a Mitzvah, we are safe.
The Ohr Hachaim has another answer for this question, why did the Torah include the ability and obligation to guard Mitzvos as part of Hashem’s elevation of Klal Yisroel.
Moshe Rabbeinu was telling Klal Yisroel that they were about to experience something that even he could not experience. Moshe Rabbeinu davened desperately to Hashem to be allowed to enter Eretz Yisroel. Chazal ask ‘did he wish to eat from its fruits? Rather, he said to Hashem that many Mitzvos depend on living in Eretz Yisroel, etc..’
Moshe was telling them that they should recognize and appreciate the privilege that they were being granted. Hashem elevated them and chose them to be able to live in Eretz Yisroel and fulfill all of the Mitzvos that are dependent on it.
For this reason, the Torah emphasizes that it is this day that Hashem chose Klal Yisroel. Now, when they are to enter Eretz Yisroel, Hashem has elevated them to keep all of His Mitzvos. This is a privilege that even Moshe Rabbeinu was not granted.
