Complex sentence: The Cathari were thought to be dualistic, believing not in one all-encompassing god, but in two, equal and comparable in status, but their theology and cosmological perception and teaching remains unknown, as no known source has survived that can elucidate this.
Split into simpler sentences: The Cathari were thought to be dualistic in faith, and that they did not believe in one all-encompassing god, but in two, both equal and comparable in status. But their theology and cosmological perception and teaching remains unknown, as no known source has survived that can elucidate this.