The Avos have an anomalous status in Jewish thought. On one hand, they are our Fathers and this makes them Jewish, right? On the other hand, until Yakov came along they all produced non-Jewish progeny. Avraham had Ishmoel and Ketura's children and Yitzhak had Eisav.
Some think that Avraham was the first Jew and some do not. The first opinion is certainly much more widespread.
Every Jewish child learns that Avraham Avinu was the very first Jew. When this question was asked on Wikipedia, it answered that it was the physical act of the bris milah at age 99 made him into a Jew. That is one of the many answers given in the traditional Torah commentaries to this question. Other contemporary answers span when he was 3 years-old to age 137 (listen about 3 minutes that he had to pass the final test of the Akeidah ) before he "officially" became Jewish.
In between these exteremes, we find the Rambam’s opinion that Avraham was 40 when he
became Jewish. (See here). Others state he was 48 or 52, the latter according to Gemara Avodah Zarah daf 9 being when the era of 2000 years of Torah began with Avraham. Other candidates for the age in which Avraham became Jewish include when he listened to HaShem at age 75. This is he age in which he received his first Mizva that he had to obey as opposed to all his earlier actions which were on his own motivation (see sicha Likutei Sichos 25,47 ). This is also the age in which he went to the
We can see that there is wide range of contemporary opinion on this question. The Rogatchover in his commentary to Torah points our that each Avraham kept on advancing levels in serving Hashem and that he reached the level of "BAKOL" when he was at this age of 52. If so, each suggestion draws on a different level in a life of constant striving and growth; however, the Rogatchover’s suggestion also means that for Avraham, Yiddishkeit was a process and not a specific station or stage. Each of the ten tests can be seen as staged transitions in his connection to HaShem. Each view above can then be seen as representing a different stage in the progression of Avraham's avodah: he was not an instant Baal Teshuvah but a growing one! Each is measured by a different physical, spiritual or halachic parameter.
In R. Yehuda Rosannes’ Parashas Derachim(Drasha 1) and in Beis Otzer from R. Yosef Engel when Avraham became Jewish is explored in halachic terms relative to being a Ben Noach.
No one disputes that for Avraham there was a clearly defined break from his father Terach’s idolatry; however, there were other great monotheists at the time, who were not Jewish: Chanoch, Metushelach, Shem and Ever, for example. Chabad Chassidus explains that these people did not have an effect on their environment: they were each a "tzaddik in peltz" . Before Avraham one could be all that he was destined to be without a connection to a people. Once Avraham arrived on the scene and had his bris, holy souls that descended to the nations of the world had to convert to Judaism to reach their full potential. Their descendents included great masters of the Mishnah such as Rebbe Akiva and Rebbe Meir. According to Chassidus via expanding the opinion in the Kuzari, introduced in Tanya 2, there is a distinct difference between a Jewish soul and those of other living things.
We can say that until his bris, Avraham was a Jewish soul in a non-Jewish body. When did this change? At his bris at age 99! This is when Avraham's Jewish soul came to reside in his Jewish body. This was clearly related to his circumcision.
For most of their life together Avraham and Sarah were physically unable to have children and this was an expression of their not yet having fully status as Jews. Once Avraham attained this state of Jewishness, his potential to spiritually affect the world ( see here and here ) in a manner unlike that of his predecessors was realized in a actuality.
There are other perspectives on this question in Chazal and Chassidus, that say that Avraham ( and other Avos) never did attain a completely Jewish status since he did not receive the Torah on Sinai and although he fullfilled all the commandments, it was not the same as after the Giving of the Torah at Sinai. Moreover, he gave birth to a Yishmael and it was not until Yaakov that all of the offspring remained Jewish.
There are many recent books exploring the life of Avraham Avinu . (see The discovery of God / David Klinghoffer. and Abraham's journey : reflections on the life of the founding patriarch / Joseph B. Soloveitchik ; edited by David Shatz, Joel B. Wolowelsky, and Reuven Ziegler. ) from various perspectives. Searching the internet, one can find many "most influential" lists of religious personalities and although one can make a good case that there were Jews who made a greater impact on our daily life today than Avraham (Moshe Rabbeinu, for example), it all started with Avraham !
1.Chidushei Hagriz ( in the back of some editions of Beis Haleivy on Chumash) points out that from the Haggada it would appear that Terach in also considered one of the Avos. The Haggada quotes: “It was on the other side of the River that YOUR forefathers dwelt a long time ago, Terach the father of Abraham and the father of Na´hor, and they used to serve other gods (Joshua 24:1)."
2.In various acharonim one finds the distinction between kedushas yisroel and kedushas hamitvos. This little piece of lomdus was said by R. Yosef Dov Soloveitchik but I remember seeing it also in acharonim to the 8th chapter of Bechoros. The Jew should have both kedushos, however even a non-observing Jew remain a Jew by the virtue of Kedushas Yisroel, with which he is born. A convert, on the other hand, has kedushas hamitzvos, assuming he had sincerely accepted upon himself or herself to perform them. However, he does not have keshushas Yisroel and is free from certain kedusha related limitations of personal status, for example, the prohibition of marrying a mamzer or a nosin.
With this principle, one can analyze the question of when did Avraham became a Jew (if ever) from the perspective of the two kedushas and gain additional insight into the the opinions that Dr. Gershon shared with us.Those opinions which relate this change of status to the commandment of Lech Lecha, or to the onset of the 2000 years of Torah or to the abandonment of idolatry by Avraham, view the element of Kedushas Hatorah V'Hamitsvos as being primarily that which defines a Jew. If on the other hand, it was kedushas Yisroel which is primary, the circumcision comes to the fore as the one bodily change that effected kedushas Yisroel.