From the Introduction to the core rulebook:
Geist: The Sin-Eaters is a game about people. Some of them are alive, some of them are dead, and some of them stand between the two, but they are all people.
Geist is about giving a voice to the voiceless, of standing up for the marginalized, about power and privilege and the responsibilities they carry with them.
Geist is a game about death.
Geist [is] a game about hope.
Sin-Eaters [...] act as intercessors and speakers for the dead, helping ghosts finish their unfinished business and laying tormented souls to rest.