Letters of correspondence between Pope John Paul II and a married Polish-born American woman reveal a close relationship which spans more than 30 years.
The letters, obtained by the BBC, reveal that in 1973, Anna-Teresa Tymieniecka wrote to the then Archbishop of Krakow, Cardinal Karol Wojtyla - who would later become Pope John Paul II - to discuss details of a book on philosophy that he had written. The letters from the Pope to Tymieniecka have been sighted by the BBC.
Pope arrives in Mexico's Chiapas state
Pope Francis arrives in Tuxtla Gutierrez, the capital of Chiapas state, part of Mexico's poor, indigenous south, where he will authorize the use of native languages for Mass in a bid to stem a tide of Protestant conversions.
After Tymieniecka travelled to Poland from the US, they continued to write to each other and in time, the relationship grew.
The rare manuscripts dealer who facilitated the sale of the letters, Marsha Malinowski, says that it's clear from the letters that Tymieniecka was in love with the Pope, telling the BBC: "I do believe she completely fell in love with him during the first phase of their relationship.
"I think that it's completely reflected in the correspondence."
Photographs reveal that the pair had also holidayed together. Tymieniecka also visited the Pope at the Vatican, however, there is no suggestion that the relationship was sexual in the correspondence.
Despite rising to the papacy in 1978, the friendship did not slow down - in fact, John Paul II gave Tymieniecka a scapular given to him by his father. A scapular is made of two tiny pieces of cloth attached by a ribbon or cord and worn over the shoulders, to remind the wearer of their lifelong devotion to their faith.
John Paul II told Tymieniecka it allowed him to "accept and feel you everywhere in all kinds of situations, whether you are close, or far away".
As the Pope's battle with Parkinson's Disease began in the 1990s, Tymieniecka visited him at the Vatican several times.
Tymieniecka's husband, prominent economist Hendrik S. Houthakker, also went on to advise the Pope on several key issues, and was himself awarded a papal knighthood in 2003 in recognition of his services.
Tymieniecka's letters to the Pope have not been recovered by the BBC, but it is believed that the National Library of Poland, who have not confirmed possession, acquired copies of the letters in 2008.
Pope John Paul II passed away in 2005 and was canonised as a Saint nine years later, in April 2014.