Good day! I hope you are doing well.
I have a friend who's a transgender atheist, though she did not surgically alter her body (since she's barely a teenager). She inwardly feels that she is a he. She has even demanded us to call her by a male name, as her sort of new alias.
Now I know that transgenderism, in general, is rather sinful. And I hold that belief.
She has come to me for advice on what to do with her gender dysphoria (her dislike of being in a woman's body while in a male mindset). I gave her advice that's, let's just say, not in accordance with God's Will. I am about to repent of that sin. When things regarding sexuality and gender overall, I think I should just send them to someone else.
Still, I'm curious, What would you do in my position? What would you say?
Since she claims to be an atheist, biblical answers won't make much of an impression, so we will approach this by pointing out the facts that God already told us.
She claims to feel like a boy. Thus, my question is: Just what does a boy feel like? What does a girl feel like? Often, the answers may strike you as odd. I know what a boy feels like because I am one. But a girl's description of what boys feel generally doesn't match up. For instance, boys tend to describe themselves in terms of actions. Girls tend to describe themselves in terms of feelings and emotions. It isn't absolute, but the trend is there.
If she claims to like boyish things, then I would ask who made the rule as to what boys or girls must like? This causes a dilemma for many people because they are assigning gender roles while at the same time denying that such roles exist.
Next, ask how many biological genders are there? The answer is only two. (See There Are Only Two Sexes). There are XX and XY chromosomes, which we label "female" and "male." While there are a few rare chromosomal defects, she doesn't have one. If there was a crime and her DNA was found at the scene, the police would rightly say that the suspect was a female. To claim there are more or that a person isn't the gender of his or her DNA is denying reality. We usually say that a person who denies reality has problems. If I insist that there is a pink elephant in the room, agreeing with the delusion doesn't make it real. While she is discontented with who she is, science shows that claiming to be the other sex doesn't solve the discontentment. See Sex Reassignment Doesn’t Work
These points are aimed to get your friend to think. They may not convince her, but they will likely bother her and perhaps later she will change her mind.
I think that one of the stronger points is that the vast majority of young people who claim gender dysphoria grow out of it. "98% of gender confused boys and 88% of gender confused girls eventually accept their biological sex after naturally passing through puberty" ["Gender Ideology Harms Children", American College of Pediatricians, 2016].