In this post I mentioned there is discussion if the Avos were Jewish or had the status of Bnei Noach. In Parshas Yisro, according to the Ramban, Yisro came and converted before matan Torah, so to him (at least) there must have been yahadus (Judaism), or more specifically, kedushas yisrael, before Har Sinai. We will see this is so, and much more.
Ramban Vayikra 24:10 discusses whether the son of Shlomis bas Divri and the Egyptian man was Jewish or not. He cites the French (i.e., Baalei Hatosfos) as saying before matan Torah, lineage followed the father, so this man was not Jewish. Ramban himself argues and holds from the time of Avraham, Yisrael was a distinct people, and certainly this man whose mother was Jewish was also Jewish, from the descendants of Avraham.
We also see this from Ramban in Yevamos 46 when the Gemara says a ger must undergo milah, tevilah and bring a korban, when the Gemara says a ger needs tevilah like the imahos (mothers), Ramban explains it refers to Sarah, Rivka, Rochel and Leah. Rashi, however, explains mothers as the Jewish women who left Egypt.
Rav Ahron Halevi Soloveichik zt"l explained (See Mesorah 19, Teves 5763) that there are two sanctities (kedushos) that a Jew has - the individual sanctity and the communal sanctity. The Avos had individual sanctity (according to all opinions). Ramban Yevamos holds the laws of geirus are learned from one acquiring the individual sanctity, which the Avos had. Rashi there learns the laws from the communal sanctity, which did not exist until Har Sinai. (And when we say a mumar loses his kedushas yisrael - that's only his communal sanctity, but he does not lose his individual sanctity.) He also explained that the argument between the Ramban and the French rabbis regarding the son of Shlomis bas Divri was whether religion following the mother was dependent on having the individual sanctity (Ramban; hence the man was a Jew) or having the communal sanctity (French rabbis; hence that man who was born before Har Sinai was not a Jew like his mother, rather an Egyptian like his father).
The Ritva (Kesuvos 11 s.v. mai) agrees with the Ramban. The gemara there discusses converting a minor (child younger than 12 or 13). He asks why don't we bring a proof that such a conversion is valid from the fact that there must have been children who left Egypt and they converted with their parents at Har Sinai? His second answer is: the seed of Avraham were already commanded to do milah and to to bring them [their children] into the covenant [of Avraham] from their youth, and this [procedure at Har Sinai] was only a completion of Geirus.
The Ritva's answer is that the Jews had individual kedushas Yisrael before Har Sinai, and they added on the communal kedushas Yisrael at Har Sinai. (It seems the Ritva in his question held like Rashi).