Pope Francis on Friday warned of the dangers of gender identity theory, also known as “gender ideology”, saying he had commissioned a study of what he called an “ugly ideology” that threatens humanity, Reuters reported.

Speaking before a Vatican conference on the changing roles of men and women and Christian teachings on the subject, Pope Francis said “gender ideology” is a threat because it seeks to erase differences between the sexes.

“I called for an investigation into this ugly ideology of our time, which abolishes differences and makes everything the same. To abolish differences is to abolish humanity,” said the sovereign Pontiff.

Gender identity theory, often called “gender ideology” by its critics, suggests that gender is more complex and “fluid” than the “binary” categories of male and female, and that it depends not only on biological sex.

Pope Francis also said on Friday that he was still suffering from a cold that had made him ill in recent days and asked an aide to read his speech, but that he wanted to “underline” his thoughts on gender ideology for the first time as he called the theory.

He reported that he remembered reading a “prophetic” book on the subject: “Lord of the World,” a dystopian novel published in 1907 by a Catholic priest that warned of the dangers of blurring the differences between people, leading to the risk of religion disappearing

“Read it if you have time, because it speaks to the problems of our time,” Pope Francis told those attending a conference in the Vatican.