Which bris made Avraham jewish

There is a well established approach within jewish tradition that pegs Avraham as the first jew. The precise event that triggered this momentous change in status is the subject of this exploration.  

Avram's words of (bereishis 15:2-3) וַיֹּאמֶר אַבְרָם אֲדֹנָי יֱ־הֹוִה מַה תִּתֶּן לִי וְאָנֹכִי הוֹלֵךְ עֲרִירִי וּבֶן מֶשֶׁק בֵּיתִי הוּא דַּמֶּשֶׂק אֱלִיעֶזֶר. וַיֹּאמֶר אַבְרָם הֵן לִי לֹא נָתַתָּה זָרַע וְהִנֵּה בֶן בֵּיתִי יוֹרֵשׁ אֹתִי furnish R Elchanan Wasserman with proof (kovetz shiurim vol 1 358) for Rambam's opinion (nachalos 6:9) that by bnei noach only a son inherits, as otherwise Lot should have been the one standing to inherit Avram as his brother's son. R Elchanan then says that even acc to Ramban (vayikrah 24:10) that the avos were Jewish (for religious identity and inheritance matters), that only occured at the bris milah which only happened later on with Avram as of yet still a ben noach.

Based on this he questions a midrashic statement quoted in Rashi (bereishis 13:7) that the shepherds of Lot left their animals to freely graze in canaan saying that as Lot is a nephew he stands to inherit the childless Avram. He ends off suggesting that (although the midrash intimates that bnei noach have broader inheritance laws than Rambam's view of sons only) maybe Avram intended to strip it from the less deserving Lot and transfer it to the more ethical and faithful Eliezer. 

עד כאן דברי ר אלחנן

If one looks at the divrei Chazal to those pesukim (Medrash rabbah 15:2, Lekach tov ibid:3) one will find an opinion that the referent of those pesukim is not Eliezer but rather Lot, thus resolving R Elchanans problem.

Midrash rabbah 

וּבֶן מֶשֶׁק בֵּיתִי – רַבִּי אֶלְעָזָר אוֹמֵר בֶּן מֶשֶׁק בֵּיתִי, זֶה לוֹט, שֶׁנַּפְשׁוֹ שׁוֹקֶקֶת עָלָיו לְיוֹרְשֵׁנִי. הוּא דַּמֶּשֶׂק אֱלִיעֶזֶר, שֶׁבִּשְׁבִילוֹ רָדַפְתִּי מְלָכִים עַד דַּמֶּשֶׂק וַעֲזָרַנִי הָאֵל.

Lekach tov 

והנה בן ביתי יורש אותי – נאמר על לוט

The issue now runs deeper:

Why would Chazal twist themselves into a pretzel to creatively read the verses as referring to Lot and not just allow them to mean—as they plainly connote—that Eliezer stands to inherit? In other words, why argue on Rambam's narrow conception of noahide inheritance law, enlarging it to include nephews, thus necessitating a creative reading of the verses, when one can simply subscribe to Rambam's position and explain the verses literally?

The answer may be that the above referenced opinion of Ramban's (Vayikra 24:10) that

 כי מעת שבא אברהם בברית היו ישראל ובגוים לא יתחשבו, וכמו שאמר בעשו (בבלי קידושין יח.) ודילמא ישראל משומד שאני

which R Elchanan took to mean as referring to the bris milah is perhaps in reality referring to the other bris Avram engaged in—the bris bein habesarim. (Although artscrolls library includes the word milah in the text of Ramban itself, the MHK edition, among others, does not include it, with manuscript evidence supporting its omission.)

Per Chazal (eg seder olam rabbah 1, bt berachos 7b as explained by tosefos there), the bris bein habesarim (described in bereishis 15:7-21) was HaShem's first contact with Avram and when He initially promised Canaan to his children. According to these sources Avram was 70 years old when he was summoned from Ur kasdim and told to go to canaan where he was promised that it would belong to his progeny and where the bris bein habesarim was performed to ratify it. Chazal say that the election of Avram was in response to Avram having been willing to be burned alive for HaShem. As such it is sensible to assume that this is the bris that changed Avrams religious status.

As Ramban himself (bereishis 26:5) famously links judaism and the mitzvos to the land of israel it is reasonable to assume that the bris he is referring to is the one bein habesarim. (Full disclosure: although Ramban personally rejects Chazals achronological rendering of bereishis 15 [in his commentary to bereishis 12:40], I dont think it materially affects the larger point as it still preceded the bris milah.)

Per Chazal, the bris bein habesarim (ibid 15:7-21) occured prior to the narrative recorded in the immediately preceding pesukim (ibid 15:1-6) which include the above mentioned ben meshek beisi standing to inherit etc. 

That being the case, Avram was already Jewish before giving voice to his concern about someone other than his progeny inheriting him (ibid 15:2-3). Ergo, the first jew's estate was already then governed by jewish law with Lot being the putative yoresh.

This explains why Chazal were compelled to creatively explain the pesukim as referring to Lot when peshuto shel mikra clearly supports Eliezer as the referent. 

It turns out that in the world of a chronologically accurate perek 15, with Avram still a ben noach, these pesukim do indeed provide proof for Rambam's opinion regarding bnei noach inheritance law, as R Wasserman posited. 

Whereas in the world of Chazal, with Avram already Jewish from the age of 70, Chazal were constrained to explain the pesukim as referring to Lot, as he—and not Eliezer—was the rightful yoresh of Avram as per jewish inheritance law. This too allowed Chazal to explain Lot's shepherds argument as reflecting a finer point of Jewish inheritance law.

After writing the above I came across R Yehudah Rosanes in his פרשת דרכים where he too assumes like R Elchanan (as does מהר"ש יפה—cited there) that it was the milah that transformed Avraham into a jew, not the bris bein habesarim. 

Maharsha as well (Bavli Yoma 28b) asserts that the bris milah is what converted Avraham.

I can think of three reasons why they're all making this assumption. 

One is that if it was before the bris milah and Avram kept all the mitzvos already then why didn't he circumcise himself already as well? See here for different suggestions. 

The second one is that if it was before milah then how did Avram become Jewish, as ostensibly milah is necessary for males to become Jewish. 

I noticed that this second point is dealt with

in the notes there; with this very question of when Avram jewish apparently a point of contention between great rabbis of years past (a third possibility—from when he figured out there was one God, at either age 3 or 48—is put forward there. The source that Avraham kept the whole Torah—Bereishis 26:5—is actually the selfsame pasuk that provides the Rabbis with the numerical allusion that he divined a divinity at age 3, with Avraham having observed the Torah for the amount of years of the  numerical value of עקב which is 172, as he lived for a total of 175 years).

A third issue may be that becoming Jewish vs mere noachide status ostensibly requires additional commandments, not just a status change.

פרשת דרכים there evinces proof that Avraham was fully Jewish (as opposed to a ben noach that also kept the Torah) from a gemara in bBB 100 that Avraham acquired Israel via kinyan chazakah which is unavailable to bnei noach with R Akiva Eiger querying him that that occured well before the bris milah which made him Jewish. Acc to the above it works out nicely as he was already Jewish prior to parashas lech lcha. 

See here for a more general discussioln regarding if Avraham was the world's first jew or if the Israelites only became Jewish at Sinai