The phrase “light to the nations” has been distorted along with most of the Torah by the Hellenizers of our day, the Erev Rav.
Before us we have a recommendation for one world and one people, and why should Israel remain alone, a nation that dwells apart?
Likewise, regarding the verse, "Return, return, O Shulammite. Return, return, that we may look upon you. What will you see in the Shulammite? As it were a dance of two companies" (Song of Songs 7:1), our sages offered the following description (Shir HaShirim Rabbah, 7:2):
The nations of the world say to Israel, "How long shall you die for your G-d and pay Him... How long shall you be killed for His sake... How long shall you show G-d kindness when He shows you misfortune? Come to us and we shall appoint you governors, prefects and commanders. 'Let us look [nechezeh] upon you--You will be the admiration [machazit] of the whole world" As it says, "You shall be seen [techezeh] of all the people" (Ex. 18:21).
The admiration of the whole world, a "light unto the nations"... Here is their basic enticement, adopted also by the Hellenists and falsifiers of Israel. They have distorted the concept of Israel being a "covenant of the people, a light unto the nations" (Isaiah 42:6). In their hands, this has turned into a demand that we depart the Land of our isolation and cling to the nations in the exile, living there with them in order to serve as their beacon. To achieve this, we are supposed to abandon unpleasant, "unacceptable" concepts and laws, lest these make the nations hate us, and all this in the name of unity. That is, we are supposed to assimilate for the sake of unity. (Or HaRa'ayon, Havdallah, Rabbi Kahane)
Being a "light to the nations" isn't a commandment or an action we perform nor our mission. It is the affect we have on the world by serving Hashem and following his Torah.
The 613 mitzvoth are called pillars of light. When a Jew fulfills one of the 613 mitzvoth he/she brings down light from above (heaven) to below (earth), as we learn from the Tanya:
And this is what the Yenuka meant when he said that "The Supernal light that is kindled on one's head, namely, the Shechinah, requires oil," that is, to be clothed in wisdom, which is called "oil from the holy anointing," as is explained in the Zohar, that "these are the good deeds," namely, the 613 commandments, which derive from His blessed wisdom. Thereby the light of the Shechinah can cling to the wick, i.e. the vivifying soul in the body, which is metaphorically called a "wick." For just as in the case of a material candle, the light shines by virtue of the annihilation and burning of the wick turning to fire, so does the light of the Shechinah rest on the divine soul as a result of the annihilation of the animal soul and its transformation "From darkness to light and from bitterness to sweetness" in the case of the righteous, or at least through the destruction of its garments, which are thought, speech and action, and their transformation from the darkness of the kelipot to the Divine light of the blessed En Sof, which is clothed and united in the thought, speech and action of the 613 commandments of the Torah, in the case of benonim. For as a result of the transformation of the animal soul, originating from the kelipat nogah, [a transformation] from darkness to light, and so forth, there is brought about the so-called "ascent of the feminine waters" to draw the light of the Shechinah, i.e. the category of the "revealed" light of the blessed En Sof— over one's divine soul [principally dwelling] in the brain of the head. Thereby will also be clearly understood the text "For the Lord Thy G-d is a consuming fire" as is explained elsewhere.
The Greeks forbid Jews from keeping mitzvoth, which brings down the supernal light. They also desired to eliminate the uniqueness of the Jewish people by destroying the separation between Jew and non-Jew. Only when Am Yisrael is separated from impurity and the impure nations and fulfilling mitzvoth are we able to bring down the supernal light and then we will be "a light unto the nations". The lighting of the oil which was lit in the Beit HaMikdash represented the intention of the Jews led by the Maccabees to be separate from the Greeks and the Hellenists and to return to mitzvot. (still trying to locate the source for this)
There is no commandment “to be a light to the nations” in the 613 mitzvot. This is because “being a light to the nations” is a result of the actions of Am Yisrael doing all of the mitzvot. So if you want to be a "light to the nations" or even better you want to bring down the light of Hashem to illuminate the world, DO MITZVOT!!!!