MEXICO CITY (CNN) -- As thousands of Roman Catholics throughout
                  North America pay tribute to Mexico's patron saint, a retired abbot is
                  casting doubt on the revered tradition that surrounds her.

                  Each year on December 12, believers remember a tradition that says an
                  olive-skinned Virgin Mary, known as Our Lady of Guadalupe, appeared in
                  Mexico to an Aztec Indian named Juan Diego in 1531.

                  According to the story, she left her image behind on Diego's cloak. The
                  apparition is credited with turning millions of polytheist Indians into Roman
                  Catholics.

                  Now, a retired abbot who served at the Mexico City basilica where the
                  Lady of Guadalupe is enshrined says the Juan Diego story may be false.

                  In a five-page letter to the Vatican, Abbot Guillermo Shulenburg wrote that
                  there are no documents proving the existence of Juan Diego.

                  Mexican clergy in the Roman Catholic church have been hoping Juan Diego
                  would be canonized early next year to become the second saint from
                  Mexico.

                  Letter throws Mexico's church into turmoil

                  Guillermo's letter has derailed Juan Diego's canonization process and has
                  thrown Mexico's Catholic Church into a turmoil.

                  The Vatican's representative in Mexico has questioned the abbot's intentions.

                   "This is a very particular situation regarding his psyche," said Papal
                   Nuncio Justo Mullor.

                  Lady of Guadalupe part of nation's identity

                  Our Lady of Guadalupe is, for many, at the root of Mexico's national identity
                  and its contemporary faith. Mexicans venerate her image, which is both a
                  religious and cultural symbol. It decorates murals, storefronts, even
                  niches in front of houses in Hispanic neighborhoods.

                  Thousands of pilgrims flock to Mexico's basilica daily to pray to an image of
                  Our Lady of Guadalupe, some crawling for blocks on their hands and knees.

                  The controversy has threatened to result in a religious backlash. "All this
                  back-and-forth, this war of accusations discredits the church as an
                  institution," said Fabiola Guarnero, a religious analyst.

                  But pilgrims still flock to the basilica by the thousands, more focused on
                  asking the Virgin of Guadalupe for miracles than the controversy that swirls
                  around her.

                   Mexico City Bureau Chief Harris Whitbeck and The Associated Press contributed to this
                                             report.