Complex sentence: Abdallah's claims to be the awaited Mahdi caused a split in the movement in 899, with the majority, including Hamdan Qarmat, rejecting the Fatimid claims and leaving to continue their proselytization elsewhere.
Split into simpler sentences: Abdallah's claims to be the awaited Mahdi caused a split in the movement in 899. The majority, including Hamdan Qarmat, rejected the Fatimid claims and leaving to continue their proselytization elsewhere.