I was thinking they hold honorary positions in the church and act as living immortal proof of their religion's power, but actually they're nothing more than a well preserved zombie and they serve knowingly or unknowingly to advance the sinister power that actually runs the organization. Some are so fervent they believe lichdom is a stolen ritual and they are the real thing, some don't remember being turned and might be disgusted to find out the truth, still others may hold this as blasphemy and seek to destroy the current church.