Complex sentence: The completion and consummation of the development of religion came about with the coming of Muhammad; and that the perfection of the "manifestation" of Muhammad's prophethood and of the conveyance of his message was destined to occur with the coming of the Mahdi.
Split into simpler sentences: The completion and consummation of the development of religion came about with the advent of Muhammad. However, the global conveyance, recognition and eventual acceptance of his message (i.e. the perfection of the "manifestation" of Muhammad's prophethood) was destined to occur with the coming of the Mahdi.