There's lots of different kinds of Christian. Perhaps you can frame it in a way that isn't too challenging to their belief system? Do a little research on whatever church they attend, and look at their attitudes toward gender, then try to find bable passages that preach tolerance toward others. Stress the medical nature of the condition, that this is just something that happens to people, and that their god has seen fit to "bless" humans with the knowledge to treat it.
If you know what denomination they are, I'm happy to try to help you with ways to explain it in a way that fits their world view
I don't believe in gods but, I've read a lot of different texts written by people who do. The challenge you face is the nature of all religion is denial of evidence that doesn't support existing belief. This is a really useful thing to know when you're trying to get someone with beliefs to agree with you, as long as you frame it in a way that strengthens that their belief is correct, they'll be on your side.
They may turn out to be totally supportive of you, so don't build it up in your mind just yet