1) Does every shia sect believe in the reappearance of Imam Mehdi aj? For instance, Imam Mehdi aj is linked to Imam Musa al Kadhem as we the twelvers follow. But what about the Ismaili Shia Muslims, do they believe the same way about the reappearance?

2) There is no rule saying that the oldest son becomes the successor and the Imam. Is there other arguments to why we have shia Muslims dividing from Imam Ja'ffar Al Sadiq and what's the differences between the twelvers and ismailis?

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1) If you a Shia ithna Ashari or even a Sunni or a Zaidi you would believe in the return of Al-Mahdi, of course with some variations, however the core belief of a Messiah coming back is even extended to Jews and Christians and even other religions.

2) As for the Ismailis they have opted to stop at the sixth Imam and from there onwards it was passed to the eldest son of Imam Jaffar Ismail and then it was a successorship without any proof of divine appointment as mentioned in the authentic hadiths that both the Sunnis and the Shias narrate that the prophet said the successors after me are 12 not 54 or 56 as per the Bahoras and the Ismailis