Transgenderism is a sincerely held personal belief, relating to how one conducts oneself. What would be the legal ramifications (US) if transgender identity was legally recognized as a religious belief?
It would expand the class of cases where discrimination is illegal, since discrimination on the basis of gender identity is only statutorily regulated in a piecemeal way. Governments could not do it at all, so laws which prohibit a person from using toilets or joining the military based on their self-identified gender as opposed to their birth gender would be unconstitutional. There are places where the Civil Rights Act of 1964 protects religion but not sex, Title II (public accommodations) and Title III (access to public facilities, though this would also follow from the First Amendment in this scenario), and gender identity would then be protected just like race is.