[Dead Sea Sunrise]
Devarim 8:1 You shall faithfully observe all the Instructions that I enjoin upon you today, that you may thrive and increase and be able to occupy the land which Hashem promised on oath to your fathers.
This verse follows immediately after the discussion of idolatry in order to teach us that whoever denies pagan worship is regarded as having upheld the whole Torah.
The verse uses the words Kol haMitzvah (כל המצוה - all the Instruction) instead of the plural form Kol haMitzvot (כל המצות - all the Instructions). A number of possible explanations are offered. The first reason is to signify that the entire body of laws in the Torah constitutes a single entity. One is not permitted to state that since he observes most of the, he can afford to omit others. One is not permitted to pick and choose among the Mitzvot since they all represent one unit.
Another reason offered is that the singular terminology does not refer to the mitzvah itself but to the performer of the mitzvah. That is to say, if one person begins a mitzvah but for some reason is unable to complete it, the mitzvah accrues only to the account of the person completing it. It is not considered a joint venture but is exclusively to the credit of the latter.
The singular use can also be understood as signaling that if any mitzvah is performed, it is to be observed with the totality and devotion as if it were the only mitzvah in the Torah. This is why pious people prepare themselves intently before performing a mitzvah - meditating upon the mitzvah and reciting the biblical texts which refer to the mitzvah they are embarking upon.
Finally, a singular usage signals that all mitzvot of the Torah constitute a single organic whole. Therefore, one who is unable to perform various mitzvot because of circumstances (such as an ordinary Jew who is unable to perform the mitzvot that involve only Kohanim), but nevertheless performs all the mitzvot that are required of him faithfully, and studies those mitzvot which are not applicable to him, receives the reward for even those mitzvot which he cannot perform.
Support for this last interpretation is found in the usage of the word Hayom (היום - this day) in the verse. Since it is evident that in the desert there were many mitzvot which the Benei Yisrael were not able to perform, particularly those dealing with the Land of Yisrael, nonetheless, they were adjured to heed all the mitzvot, even those which fell into this category.
- Me'am Lo'ez, Devarim, Parashat Ekev