The Torah commands many times to give special treatment and love to gerim (converts). Many instances apply even to a ger toshav, for example in our parasha: "If your brother’s status is lowered and his means of support falter, you shall strengthen him, whether he is a convert or a resident (toshav) and lives among you" (Vayikra 25:35). Who is this toshav? Rashi explains: "It is someone who has accepted upon himself not to worship idols, yet he still eats neveilot (improperly slaughtered animals)."
According to one opinion (Avoda Zara 64b) a ger toshav is someone who accepts all the mitzvot of the Torah except for eating neveilot. Our mentor, Rav Shaul Yisraeli z.t.l. (in Chavot Binyamin 67), asked: Why would someone who is willing to accept almost everything not go a step further and become a ger tzedek (a full convert)? Let us introduce two issues before trying to answer the question. Chazal tell us that a convert has a status of a newborn child, a categorization that applies to lineage in regard to the laws of incest. This concept, whose source is far from clear, applies to converts over the generations but did not apply to Bnei Yisrael at Sinai, even though they underwent a conversion process at the time.
The process of conversion has three parts: accepting the mitzvot, circumcision (for men), and tevilla (immersion). According to Tosafot, only the former requires a beit din. Rav Yisraeli explains the process of acceptance before beit din and many other things as follows. In order to convert, one must be accepted into Bnei Yisrael by representatives of the nation (i.e., a beit din). Beit din is authorized to do so only if the candidate is willing to accept the mitzvot; once he is told about some of the mitzvot and accepts all mitzvot, his obligation in them is not the result of his acceptance of the mitzvot but of his acceptance into k’lal Yisrael. This is the idea behind the most famous convert’s statement: "Your nation is my nation, and your G-d is my G-d" (Ruth 1:16).
The conversion process begins with an act of national acceptance. A non-Jew’s independent acceptance of mitzvot is meaningless; conversion without national acceptance is not conversion. As one changes national affiliation, he assumes a new identity regarding lineage while keeping his identity regarding personal matters. When Bnei Yisrael converted at Sinai, they did so as a nation together and thus the concept of new lineage did not apply. One who embraces the precepts of the Torah but is unwilling to change national identity can be a ger toshav, not a ger tzedek.
Ramifications of this thesis include that while conversion can occur only for one who accepts mitzvot, his or her failure to keep them afterward does not undo the conversion. Also, one’s interest in being part of the nation is an important consideration for beit din in determining who is fit to be a ger.