Mishpatim
The Ben Noach Versus the Ger Toshav (based on Chavatzeles Hasharon pp. 632-640)
The Bnei Noach, although not Jewish, are obligated in seven mitzvos (1). If a non-Jew accepts these mitzvos before a Beis Din, he becomes a Ger Toshav (Mishnah Torah, Hilchos Melachim 8:10).
The verse says, “On the seventh day, rest, in order that your animals be given rest, and the child of the servant and the Ger” (Shmos 23:12). Rashi writes that the ‘Ger’ here refers to the Ger Toshav, not a full convert. This is a Chazal in Yevamos 48b and in the Mechilta. In the Gemara in Krisos (9a) there is an argument what Melachos of Shabbos the Ger Toshav must refrain from.
Rashi in the Gemara (Yevamos 48b) says that since the Ger Toshav is commanded to refrain from Avoda Zara, he must keep Shabbos — since breaking Shabbos is akin to Avoda Zara.
Tosafos questions this. 1. The Ger Toshav accepts upon himself the seven mitzvos of Bnei Noach; these seven don’t include Shabbos. 2. A Ben Noach — non Jew — is not allowed to keep Shabbos! (Sanhedrin 48)
The Ponevez Rav explains (Kuntres Divrei Harav). The acceptance of the seven mitzvos of the Ger Toshav is a kind of geirus (conversion). When the Ger Toshav accepts the seven mitzvos he loses the din of Ben Noach and observes the prohibition of Avoda Zara as a Jew would, from the commands of the Torah. This would require keeping Shabbos.
Yes, the Ben Noach is not allowed to keep Shabbos, but the Ger Toshav is obligated to avoid Avoda Zara just like a Jew is — due to kabalos Hatorah — and therefore he must keep Shabbos as well.
This concept, that the Ger Toshav has a partial geirus, has been stated in several places. Rebbenu Gershom states so clearly in Krisos 10b.
The Rambam writes something amazing (Melachim 6:4). The Torah requires us to fight with the Knanim and Amalek. This is only if they have not accepted the seven mitzvos Bnei Noach! The Raavad, in his critical notes to the Rambam, challenges this. In response, the Kesef Mishnah writes that it’s possible to say that after accepting the seven mitzvos, the new Ger Toshav loses the status of Knaan or Amalek. (Heard from Rav Roizman of Phoenix)
The Brisker Rav (parshas Va’eschanon) actually holds that, since Bnei Noach are obligated in the seven mitzvos anyway, there is not a fundamental change when they accept the mitzvos in Beis Din. However, the Chavatzeles Hasharon shows that the Ohr Sameach does not agree. Further, the Rashba (Toras Habayis, Bais 5 chapter 4) writes that the Ger Toshav is like a newborn child, indicating that his formal acceptance is a form of conversion.
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1.The seven mitzvos are: avoiding Avoda Zara (Idolatry), not to curse Hashem’s name, murder, commit adultery, steal, eat a limb from a living animal, and to have a court system.
Teshuvos HaRama (Simon 10) writes that Bnei Noach would have to study tractates Bava Kama, Bava Metzia and parts of Bava Basra!
The Chofetz Chaim in Biur Halacha (304:3) writes that a Ger Toshav can accept any additional mitzva, and becomes obligated to keep these additional mitzvos as well.