Complex sentence: The reaction against the establishment of the new Church was "swift and negative," especially from the Karlovsty Synod (ROCOR), with whom the Metropolia had broken ties shortly before in 1926 and who viewed itself as the Metropolia's rightful canonical authority.
Split into simpler sentences: The reaction against the establishment of the new Church was "swift and negative," especially from the Karlovsty Synod (ROCOR). Shortly before in 1926, the Metropolia severed ties with ROCOR, but ROCOR still viewed itself as the Metropolia's rightful canonical authority.