I have some perspective on this. I'm not a christian, thankfully. I'm Jewish. So most of my experience is there. It's similar to christianity ideologically, especially in orthodox circles, but there are some big differences that make the two faiths incompatible.
There's this blessing that was written in the first century, that orthodox Jews say every morning. The conventional understanding of it, is that you're thanking god for not making you a woman. It's often written off by people, namely women as a throwback to an ancient ideology that's mostly dead today, in a postmodern world where women, for the most part, have the same basic freedoms and rights that men do. But that's an artifact of the translation.
I've always thought it was interesting, what a modern language like english does to ancient texts, often translating the words, but not the real meaning. There's so much tied up in the subtle nuances of a language like hebrew or aramaic that just gets thrown in the toilet the minute you translate it. And you have to, because the grammar is completely different. You can't, in english, wake up and thank god for not doing you into a woman. It wouldn't make sense. So you've got two options with the translation. The first, is to say "thank you god for not making me a woman," which is the one scholars and translators went with. Or, you could translate at as "Thank you god, for not transforming me into a woman." And that would be the correct translation, given the way the words are linked together, and the context of the block of blessings this one is in.
And that's all very acedemic. But it probably means that first century Jews, and by extension early Christians were aware of transgender people, even then. When you have a hormonal imbalance (like I did) that sparks a biological transition, it's nothing fun. It hurts. The existence of a blessing like this shows that there was knowledge of it, and that people approached transgender people with empathy, if not probably some amount of fear. If it wasn't common enough to matter, it wouldn't be in the prayer book.
There are also a couple of other examples in Jewish history and folklore. Like the 13th century rabbi who grew breasts to feed an infant after his (no doubt much younger) wife died. And that's widely retold as a miracle.
As far as the bible... nothing. It's just not in there.
A lot of the same arguments evangelical christians use to justify their anti-trans bigotry are based in exactly the same passages used by extreme christian groups to justify not using the medical system, not vaccinating their children. For a while in america, there was a christian anti-dentistry movement that used, again, all of the same passages. And it's fair to point out that these pieces of theological wisdom are so vague, that if taken this far outside of context... they could mean anything.
There is no anti-transgender content in the bible or anywhere in the commentary literature. Period. There's also no outright homophobia in the bible that isn't directly related to pedophelia by context. Even after Rome showed up and introduced the idea, they still struggled to understand it. You have to understand how simple these people were.
Taking their books, and stretching them beyond recognition to address the problems or concerns of today is silly, pointless, and I would argue incredibly dangerous.
But what do I know?
