Faithful Christians regard transgenderism as a denial of God's design for human nature. In this post I'll field a potential philosophical/theological objection to that position. I haven't actually run across this objection. But if it was raised, it might leave some Christians stumped, so I will take a whack at it.
Traditionally, Christian theologians have been dualists. That's in large part due to the traditional understanding of the intermediate state. On this view, when a human dies, what that really means is their body dies. But they have an incorporeal soul which survives. And the soul is the "seat" of human personality (mind, consciousness).
Depending on the philosophy that sponsors it, you have varieties of Christian dualism. Augustine was more Platonic, Aquinas was more Aristotelian. I myself am a Cartesian interactionist.
Now here's the challenge: a "progressive Christian" might object that if the soul is the essence of the person, then our body doesn't define our gender. If substance dualism is true, then gender is either grounded in the soul rather than the body, or vice versa. In either case, they are separable.
If gender is psychological, then the body is incidental. Conversely, if gender is physical, but the soul is your core identity, then gender is incidental to who you reallyare.
So this poses a prima facie dilemma for Christian dualism. Let's scrutinize that argument:
i) Some professing Christians are physicalists. For them, gender is either physical or socially constructed. If the former, then if a human is identical to his body, that relieves the dilemma. If the latter, that, too, relieves or relativizes the dilemma.
However I think physicalism is false. There are theological, philosophical, and scientific objections to physicalism. I won't rehearse them. My point is simply that I don't think that's a viable solution to the proposed dilemma.
ii) There's some ambiguity over what it means to be "essentially" human. For instance, I used to be a teenager. That was a temporary state. But while it lasted, that was a genuine part of who I was. And that was an important part of who I was.
This goes to the ambiguities of personal identity. How can I be the same person if I change?
When I was a teenager, that was a defining aspect of my identity. Even though I can still be me after I lose that property, that doesn't mean it was extrinsic to who I was at the time. To be a teenage boy is, among other things, to experience the world as a teenage boy, perceive the world as a teenage boy. To feel a certain way. To have that particular viewpoint. Although it's temporary, it's just a real, runs just as deep, as being a middle-aged man.
By parity of argument, even if (ex hypothesi) gender identity is separable from who we are, that doesn't make it extrinsic to who we are.
ii) Apropos (i), one could argue that even if (ex hypothesi) gender is grounded in the body, so long as we are embodied agents, that's a natural, basic part of who we are. To be an embodied agent is to be a gendered agent. For a human to be a disembodied agent, although that's metaphysically possible, is an artificial condition. A defective condition.
iii) Suppose that gender is grounded in the soul. Each soul is innately gendered. Even though gender has a physical expression, gender also, or fundamentally, consists of character traits. Stereotypically masculine or feminine character traits.
That model might seem to grant the dilemma. If that's the case, then gender isn't defined by genetics or anatomy.
Mind you, as I've already endeavored to demonstrate under (ii-iii), that's simplistic. But there are other difficulties with that conclusion:
That carries the tacit admission that there's something wrong with gender dysphoria. A mismatch between psychology and physiology, viz. a man who feels trapped inside a woman's body. That's an aberration. That's not the way it's supposed to be.
Even though it's not morally wrong, it's like a genetic defect or mental illness. It may not be culpable, but it can be a glitch. Not a design flaw, but flawed execution.
However, transgender apologists refuse to concede that. Rather, they wish to say transgender identity is a legitimate variety along the gender spectrum. Admittedly, there are tensions within the trans community. After all, folks who undergo hormone therapy, plastic surgery, and/or sex-change operations to relieve their sense of gender dysphoria clearly think there's something amiss.
On the other hand, transgender apologists are like people who resent being classified as disabled. For instance, you have members of the deaf community who oppose cochlear implants because that suggest there's something wrong with being deaf. Even though it isn't wrong in the moral sense, they still bristle at the implication that their condition is "defective."
iv) I'd add that this can have a moral dimension. Here we need to distinguish between morality and blame. Suppose a brain cancer patient develops sociopathic urges. Even though we don't think he's blameworthy for his sociopathic urges, it's still the case that what he wants to do to other people is objectively evil.
Likewise, people with senile dementia may say or do inappropriate things. Even though, in their state of diminished responsibility, we don't consider their conduct blameworthy, it is still inappropriate.
v) In addition, even if (ex hypothesi) the soul is innately gendered, that doesn't mean the body is a secondary consideration is the assignment of gender. To the contrary, we'd expect God to design human nature such that gendered souls have matching bodies. It is fitting for a masculine soul to have a masculine body; fitting for a feminine soul to have a feminine body. Body and soul are complementary.
As embodied agents, our bodies are the primary medium through which we express ourselves. A masculine soul will have a corresponding physical expression, while a feminine would will have a corresponding physical expression. A male body that exemplifies masculine character traits or a female body that exemplifies feminine character traits. You put a racing car motor in a racing car body.
vi) To take a comparison, suppose you had an alien species with a particular artistic sensibility, for music or the fine arts. It would be fitting for God to give them a visual system or auditory system suited to their psychological appreciation for certain sounds, colors, and combinations thereof.
vii) Now let's consider the alternative. What if souls are initially genderless. Gender-neutral. A blank slate, gender-wise.
That doesn't mean gender is grounded in the body. That doesn't mean gender is inessential to our psychological makeup.
Rather, that might mean gender is an acquired characteristic. When the soul is united to a male or female body, that begins to condition the soul. The soul experiences the world via a male or female body. That's how the soul perceives the world.
Even if gender is initially a physical property, yet as a result of psychological conditioning, that becomes a psychological property as well. Saturation exposure to a gender-specific embodied experience fosters corresponding character traits. You think like a man or woman.
viii) Moreover, this can be permanent. It becomes integral to your personality. Some experiences are transformative. To be a parent or a spouse can effect an irreversible change in your personality.
ix) When you die, when the soul is "separated" from the body, those acquired characteristics carry over. That's indelible imprinted on the soul (as it were). Who we are is the result of nature and nurture.
x) Likewise, the intermediate state is not the final state. It's temporary. We were meant to be embodied agents, and we will resume our physical experience at the general resurrection. So that's not ancillary to who we are. Rather, that's an essential feature of human destiny-for better or worse.