A Magistrate doesn't have to be a Light, though. It could be anyone, like, even a Luminary.
A Magistrate could be anyone in good standing in the church. Highly secular ranked nobles preferred. I think we have settled on three, but we need confirmation, or at least a comment, by more of the Elders. If it doesn't happen soon, though, Brance will just go ahead with it, and consequences be damned.