mistake rule

yes, that makes sense - so the idea is that someone struggling with gender dysphoria should just live with it and not treat the gender dysphoria ...

(CW: Biblical quotes, transphobia ahead)

Though, I don't find a scriptural basis for contemporary transphobia.

As far as I can tell, the clearest condemnation from the Bible is from Deuteronomy 22:5

A woman shall not wear a man’s apparel, nor shall a man put on a woman’s garment; for whoever does such things is abhorrent to the LORD your God,

but I'm not sure scripture defines what is considered a man's garment vs a woman's, and most Christian women wear pants which until recently was considered only a man's garment. Historically, men wore skirts before women did, and likewise with heels - skirts and heels were both originally men's clothing, only later did they become women's clothing. Either way, cross-dressing seems like the most overtly forbidden aspect from a scriptural argument.

Either way, there is no scriptural argument for why a trans woman shouldn't take estrogen. Sometimes Christians will appeal to Genesis 1:27:

So God created humankind in his image, in the image of God he created them; male and female he created them.

This can be interpreted as enbyphobic and as denying the reality of agender folks, etc. - but it doesn't expressly forbid a trans woman from taking estrogen, nor does it explicitly rule out gender expansive people.

In fact, in early Midrash viewed Adam as an androgyne, as having male and female parts, since God created Adam first and split the woman from him later, and since God made man and woman in his image, it is assumed God is both male and female.

A Christian might try to argue that Genesis is giving a rigid definition of humanity as only man and woman, and that this implies that men should only be like men or can't become women (and vice versa), but that still doesn't forbid taking estrogen either, nor does it prove a binary trans woman isn't a woman. I wouldn't assume a Christian believes the estrogen changes the essence of the person, and they will believe a man on estrogen is still a man.

Besides, Genesis just said God created mankind in his image, as male and female - God creates lots of things that then change later - we don't argue that God made trees and now we can't use them as lumber to make a house, a tree can become a house. An apple can become a pie, wheat can become beer, etc. Lots of animals God created change their sex, and there's nothing explicit in Genesis that denies this, let alone denies its possibility for humans.

If anything, the Bible gives mixed messages on gender-diverse people.

For example, Deuteronomy 23:1 expressly forbids anyone who has had their testes removed (i.e. eunuchs) from being part of Israel:

No one who has been emasculated by crushing or cutting may enter the assembly of the Lord.

But then this is sorta reversed in Isiah 56:

For this is what the Lord says:

“To the eunuchs who keep my Sabbaths,

who choose what pleases me

and hold fast to my covenant—

to them I will give within my temple and its walls

a memorial and a name

better than sons and daughters;

I will give them an everlasting name

that will endure forever.

...

And for Christians, in Matthew 19:12, Christ himself said:

For there are eunuchs who were born that way, and there are eunuchs who have been made eunuchs by others—and there are those who choose to live like eunuchs for the sake of the kingdom of heaven. The one who can accept this should accept it.

The idea of "eunuchs who were born that way" indicates this was probably how they at the time thought of some intersex individuals and other gender-expansive folks, as eunuchs.

And the context of the quote is that Christ is responding to the question of whether it's better not to marry, and in response he says some choose to live like eunuchs for the sake of the kingdom of heaven, and that those who can should do so. Basically this sounds like a preference for not marrying and reproducing, and living like the eunuchs, and this also connotes a positive association with eunuchs.

Then in Acts 8:36-38 a eunuch asks what prevents him from being baptized, and then was baptized without issue:

As they traveled along the road, they came to some water and the eunuch said, “Look, here is water. What can stand in the way of my being baptized?” And he gave orders to stop the chariot. Then both Philip and the eunuch went down into the water and Philip baptized him.

In summary

• the Old Testament (OT) says cross-dressing is abhorrent to God
• OT also says eunuchs cannot be part of Israel / God's chosen people
• OT also says in the new Israel eunuchs will be given a special place (an unfulfilled prophecy)
• the New Testament (NT) says people should live like eunuchs and not marry
• NT includes a scene where a eunuch is baptized

So it's a bit of a mess, to be honest.

Shame their god isn't more powerful than their devil