According to the gemoroh in kedushin Esav was a yisrael mummer. If so, we have to ask why his children are not metzuvah? How can "ki beyitzchok" solve the problem.
Rabbi Beilin
The Gemara in Sanhedrin (59b) says that descendants of Eisav are not obligated in Mitzvas Milah because the commandment of Milah was given to Avraham and his children (Bereishis 17:9) and the Torah says b'Yitzchak which implies that not all the descendants of Yitzchak are called the descendants of Avraham. The Rambam (Hilchos Melachim 10:7) explains that since Yitzchak said to Ya'akov "v'Yitein Lecha Es Bircas Avraham Lecha u'l'Zaracha" we see that the part of Yitzchak's descendants that are considered to be children of Avraham are the ones that are descended from Ya'akov.
Dov Freedman
I knew what you answered from the outset. The question is then how could Esav be considered a yisrael mumer. There is a contradiction between the two texts.- the one in Kedushin and Sanhedrin. Read carefuly my question and you will see this is what I meant.
Eisav is a Yisrael Mumar because at the time of his Bris it was not yet defined who was the Zera of Avraham and who was not. This only became clear later as explained above from the Rambam. The clarification did not affect those that were already called Yisrael (see Griz (stencil) Toldos (25) and Shiurei Reb Shmuel Kidushin 254 DH v'Hanireh). The children born to Eisav after Ya'akov was selected and Eisav was rejected are not called Yisrael.
Alternatively we may say that Din Yisrael and Zera Avraham are not dependent on one another. Eisav was considered a Yisrael (see Yavetz Kidushin 18a) he is, however, not called Zera Avraham. The Drashah of the Gemara in Sanhedrin is to exclude him from being called Zera Avraham even though he has Din Yisrael since he was physically the son of Yitzchak and Rivka.
See also Reb Tzadok ha'Kohen (Takanas ha'Shavin 19a) who explains that Eisav is called a Yisrael Mumar because he began with potential to be called a Yisrael but when he chose to becaome a Mumar he lost the oppotunity and is no longer considered a Yisrael. Apparently he learns that the Gemara in Kidushin means to say that since he started off as a Yisrael he can have a Yerushah even if other non Yisrael do not.
Dov Freedman