From the introduction:
Their real name for themselves is Eklesia, but they call each other passengers. As long as the tradition goes, they have been on a voyage on the cosmic sea towards their future home world, Elysion. They travel in small vessels each containing one passenger, only connected through a loose network allowing the transport of log entries not unlike messages in a bottle. Their knowledge of their history or customs has faded a long time ago, leaving only ghosts.
Weary of their long journey, they decided to make a stop on one of the worlds on their course. So, they landed on Earth.
They could not survive its atmosphere on their own. Instead, they had to use human bodies as hosts for their existence.