The Vatican has released a new document challenging modern conceptions of gender identity.
The 31-page teaching guidance, released on Monday, is titled Male and Female He Created Them.
It speaks of an "educational crisis" and says the current debate can "annihilate the concept of nature" and destabilise the family institution.
The document, released during Pride month, has drawn immediate criticism from LGBT groups.
It was issued by the Congregation for Catholic Education as teaching instruction to those who work with children.
It is not signed by Pope Francis himself, but quotes him and scripture within its reasoning.
What does the document say?
The document calls for dialogue, but issues guidance on a number of topics including the transgender community.
Notably it criticises the modern understanding of gender as being more complex than the binary division of sexes.
It says current theories "move away from nature" and instead "towards an absolute option for the decision of the feelings of the human subject".
It also says moveable sexual identities are "often founded on nothing more than a confused concept of freedom in the realm of feelings and wants".
It goes on to say that gender is not decided upon by individuals but imputed by God: "The Holy Scripture reveals the wisdom of the Creator's design, which has assigned as a task to man his body, his masculinity and femininity."
Analysis: Questionable timing
By Megha Mohan, BBC Gender and Identity correspondent
It came without warning and at a questionable time - LGBT Pride Month. Which is also 50 years since the Stonewall riots of 1969 - one of, if not the, most pivotal moments in gay history.
The 31-page document was decisive in its definition of the gender binary, condemning any deviation. It states that a person claiming identity beyond cisgender (when personal and gender identity is the same as birth sex) is trying to be "provocative".
The document came as a blow to activists, who say that such a stance will only alienate vulnerable people from the Church in a time where a lot of modern society has acknowledged, embraced and are celebrating LGBT communities.
In 2019 there are new and crucial conversations to be had on those whose identity varies from their biological sex: bathroom facilities, women's shelters, even prisons. But with the release of this document, the Catholic Church seems light years away from that dialogue.
The document says that institutions should teach coherently on "the true nature of human persons" on the basis of "clear and convincing anthropology".
However, it also says that children and young people should be taught to respect every person so that no-one suffers bullying or other discrimination.
New Ways Ministry, a US advocacy group for LGBT Catholics, has described the document as a "harmful tool" that will be used to "oppress and harm not only transgender people, but lesbian, gay, bisexual people, too".
Francis DeBernardo, executive director of the group, rejected the notion that people "choose their gender".
In a statement, he said the document will "confuse those who sincerely struggle with questions of gender identity and sexual orientation" and described the Vatican as being in the "dark ages" on this issue.