Twenty religious leaders, including four Catholic bishops, penned an open letter speaking against transgender people. The letter, titled "Created Male and Female," was posted to the U.S. Conference of Catholic Bishops’ (USCCB) official website.

“We come together to join our voices on a more fundamental precept of our shared existence,” the letter reads after reaffirming its opposition to same-sex marriage. “Namely, that human beings are male or female and that the socio-cultural reality of gender cannot be separated from one's sex as male or female.”

The letter, dated Dec. 15, goes on to claim “children especially are harmed” by “gender ideology” and calls on parents, medical institutions and states to fight against the "deeply troubling" and "false idea" that "man can be or become a woman or vice versa."

Marianne Duddy-Burke, executive director of LGBTQ Catholic group DignityUSA, slammed the open letter and said those who signed “represent the most extreme right wing of the U.S. Catholic Church.”

“This is a heinous and immensely damaging letter that we condemn in the strongest terms,” Duddy-Burke said in a statement sent to NBC News. “At best, those who have signed and promoted it do not appreciate the tremendous harm that it could inflict. At worst, they are cynically and callously disregarding the emerging scientific and social understanding of transgender people and their identities in the service of outdated and inaccurate ideas.”

Duddy-Burke said the letter could cause “immeasurable” harm to transgender people, their families and their communities, and she is calling for the USCCB to disavow the letter and take it off its website.

Among those who signed the letter are representatives of the Catholic, Presbyterian, Baptist, Greek Orthodox and Islamic faiths.

FOLLOW NBC OUT ON TWITTER, FACEBOOK AND INSTAGRAM