Complex sentence: The film's premise that there is another way to heaven than through the practices of the Roman Catholic Church through the intervention of the secular Sin Eater who removes all taint of sin, no matter how foul, from the soul just before death.
Split into simpler sentences: The film's premise that there is another way to heaven than adherence to the practices of the Roman Catholic Church. A secular Sin Eater, according to the legends put forward in the film, can remove all taint of sin, no matter how foul, from the soul just before death.