It was just any other day, another testament to the mundane dreary lives of the living. No one was prepared for their coming, how could they be? They entered the bodies of the living, transforming them into blood thirsty monsters, all in an instant.
The religion believes in reincarnation, but that you will be reborn in the past. People of today are those re-born of tomorrow, with some discarded as trash. The most pure will be eventually reborn in the mythical paradise-like past, where people lived for hundreds, or maybe even thousands of years.
This goes nicely with beliefs into foresight (you know a little about the future because you have been there), but also to many complaints of how things are going always from bad to worse.