Who are the Knights of Malta? The organization turns 900 at the Vatican (PHOTOS)

The Knights of Malta turned 900 Saturday, and around 4,500 or so members and volunteers celebrated with an elaborate procession through St. Peter's Square and a mass at the Vatican.

But who exactly are they? 

Dubbed "one of the most peculiar organizations in the world" by the Associated Press, the group is part Roman Catholic religious order, part humanitarian aid group, part sovereign entity. 

Formed in the Middle Ages as a hospital for sick pilgrims in Jerusalem and recognized by Pope Paschal II in February 1113, the order "no longer fights its battles with swords, but with peaceful tools against disease, poverty, social isolation as well as protecting the faith," BBC News reported. 

It is the only surviving medieval order of its kind, according to the current Grand Master of the Order, Matthew Festing, a prince and ranked Cardinal from Britain who has been elected for life. 

"It's interesting that a small band of Crusaders has expanded into this huge worldwide organization," he told the BBC. "The reason we've survived is because we have changed from knights in armor to what we are now in the 21st century. We are still doing what we did then, looking after the sick."

The Knights of Malta, which boasts 13,500 members, have no country to call their own, but act as a sovereign entity, issuing stamps, license plates, and even passports, according to the AP. They also have observer status at the United Nations. 

“Wherever we operate, we are builders of peace in a kind of singular humanitarian diplomacy,” said Jean-Pierre Mazery, the order's foreign affairs minister, Radio Vatican reported.

"We do not depend on anyone, we do not defend territories, we do not take part in conflicts, we act only to help people, regardless of nationality, race or religion," he added. 

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