I'm not out to my mom directly. But she used to police my gender when I lived at home, and wouldn't let me out of the house if I looked "too much like a lesbian." But since then she's made progress, and actually bought be men's clothes this Christmas

So, while this is a minor story, as she only knows about me liking girls and my "cross" dressing, it is something positive, especially considering how religious she is.
I would recommend explaining transgender issues; the distinction between sex, gender and orientation; and how this relates to you and your own identity and feelings, in edition to suggesting outside resources. Maybe even pose the problems of gender essentialism and how it restricts large portions of society, how these prejudices oppress you personally, and how people can be mindful and accepting. I think it's usually a good idea to emphasize how these things relate to you, not only the issues at large.