millerites...

The Millerite tradition is a diverse family of denominations and Bible study movements that have arisen since the middle of the 19th century, traceable to the Adventist movement sparked by the teachings of William Miller.
From Wikipedia...

This just seems like a group of people, with different religious beliefs. Like a cult, but they didn't really do anything bad. They just thought that Jesus would come back on a particular day, which he didn't, then the people went back to their everyday lives. There are still people who are in this faith, but call themselves something else.

Whatever, if they want to believe this, they can.