The commentary says; B'nai Yisrael would be kohanim [priests] to every person on earth, teaching them about G-d. Had it just said, "you shall be "ministers" it would imply that after receiving the Torah they must disperse to the four corners of the earth as ministers of G-d and His messengers, their task would be to teach wisdom to those who had strayed, to make man, the culmination of creation, know his Creator. However the nations of the world did not wish to receive the Torah from G-d, and surely would not have accepted even a part of it from individual Jews approaching them to teach them Torah and mitzvos. Their "reward" for their moral lessons would be annihilation and permanent enslavement at the hands of these nations. "G-d's ministers" would be prey to every wild man and despot on the earth. Israel was therefore told, "you shall be to Me a "kingdom of ministers". It would be the collective duty of this kingdom to set an example to all kingdoms on earth. It's pure faith in G-d of heaven and earth, its upright and true laws, its statutes, fair to citizens and foreigners alike, and the justice that would reign in this kingdom, would provide the nations of the world with a standard with which to conform.
(Insight's to the Torah, Rabbi Zalman Sorotzkin)
This is also the true definition of the term "light to the nations". When we (as a people) serve Hashem through the mitzvot as proscribed by the Torah, we are an EXAMPLE to the world of how to serve Hashem. "ואתם תהיו-לי ממלחת כהנים, You shall be to Me a kingdom of ministers" is a statement not a command and it can only be done when Am Yisrael is a גוי קדוש, holy nation
Being a "kingdom of priests" and a "light to the nations" does not mean we are to go out into the world and missionize the non-Jews.