Rashi explains that this section was proclaimed when all the people were assembled because most of the fundamental laws of the Torah are dependent on it. Many other mitzvos apply to only a particular segment of the nation, such as kohanim, leviim, the first-born, the males, etc.. The call to holiness, however, includes all of Klal Yisrael. Each person is implored to sanctify himself to the best of his ability. Each and every Jew has their own individual uniqueness which should be nurtured, developed, and sanctified l’shem shomayim, for the sake of Heaven.
As Horav Eli Munk, z.l., notes, the presence of the entire Klal Yisrael demonstrates that the ultimate goal of holiness can be achieved only through the collective efforts of the entire nation. The relative degrees of holiness attained by individuals are based upon the many mitzvos that are dependent upon community participation, i.e. congregational sacrifices, the gatherings on Yom Tov, Shemitah, Yovel, and the duties regarding social welfare. To achieve the highest plateau of spiritual perfection one must commune with others.
Let us expand on this concept. Holiness, in religious vernacular, represents the highest form of spiritual perfection. This achievement had previously been granted to unique individuals who devoted themselves to a life of virtue and moral perfection. For the first and only time we note here an injunction addressed to an entire nation to be holy. Indeed, we had already been adjured to be a kingdom of priests and a holy nation. Throughout history we find many holy individuals, but never a perfect nation. This is Klal Yisrael’s distinction — to be a goi kadosh, holy nation. What is generally viewed as acceptable and even proper by other nations is frowned upon and deemed undignified when it involves Klal Yisrael. Kedushah is our mandate and the national legacy by which we distinguish ourselves from other nations.