Addressing participants of a two-day conference on the changing role of men and women according to Christian teachings at the Vatican on Friday, Francis said what he called "gender ideology" is a threat because it seeks to erase the difference between the sexes.

"I asked for studies to be carried out on this ugly ideology of our time, which eradicates differences and makes everything the same," the Pope said, adding that "to eradicate differences means to eradicate humanity."

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Gender theory, often called gender ideology by its opponents, argues that gender is more complex and fluid than the binary categories of male and female and depends on more than just visible sex characteristics.

The 87-year-old pope said he was still recovering from a recent cold and had asked an aide to read his speech but first wanted to "underline" his feelings on gender ideology.

He said he remembered reading "The Lord of the World", a dystopian novel written in 1907 by a Catholic priest about a world where religion has no place, warning against the risk of eliminating differences between people.

"Read it if you have time, because it talks about these problems of our time," the Pope told his visitors.