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A century ago, three shepherd children claimed to have visions of the Virgin Mary.

The two girls and a boy said she told them three secrets, which were later interpreted as foretelling the Second World War, the rise and fall of Communism and the death of a pope.

Pope Francis is due to arrive Friday in their Portuguese hometown of Fatima. He will canonize two of them, while the third child is also on her way to becoming a saint.

Up to a million people are expected to attend the canonization Mass on Saturday.

Their visions marked one of the most important events in 20th century Catholicism and have become known as the "Three Secrets of Fatima."

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Wax figurines are placed as offerings, as pilgrims attend the 99th anniversary of the appearance of the Virgin Mary to three shepherd children, at the Catholic shrine of Fatima, Portugal May 12, 2016. REUTERS/Rafael Marchante

A pilgrim walks on her knees to fulfil her vows, as pilgrims attend the 99th anniversary of the appearance of the Virgin Mary to three shepherd children, at the Catholic shrine of Fatima, Portugal May 12, 2016. REUTERS/Rafael Marchante

A pilgrim lies prostrate as she walks on her knees to fulfil her vows, as pilgrims attend the 99th anniversary of the appearance of the Virgin Mary to three shepherd children, at the Catholic shrine of Fatima, Portugal May 12, 2016. REUTERS/Rafael Marchante

Pilgrims attend a candlelight vigil at the Catholic shrine of Fatima, Portugal May 12, 2015. Thousands of pilgrims are on their way to the Fatima Shrine to attend the 98th anniversary celebrations of the first appearance of the Virgin Mary to three shepherd children in 1917. REUTERS/Rafael Marchante

Pilgrims arrive to attend a candlelight vigil, during the 99th anniversary of the appearance of the Virgin Mary to three shepherd children, at the Catholic shrine of Fatima, Portugal May 12, 2016. REUTERS/Rafael Marchante

FATIMA, PORTUGAL - MAY 12: Spanish nuns chat as another reads 'Papa' magazine while they wait for they arrival of Pope Francis at the Sanctuary of Fatima on May 12, 2017 in Fatima, Portugal. Pope Francis will be attending the Sanctuary of Fatima, in Portugal, on May 12 and 13 to canonize two Portuguese shepherds, Jacinta and Francisco Marto, who are said to have witnessed the apparition of what they believed was the Virgin Mary, together with their aunt Lucia Santos, during the 100 anniversary. Thousands of pilgrims and worshippers from around the world are expected to gather at the centenary celebration. (Photo by Pablo Blazquez Dominguez/Getty Images)

FATIMA - PORTUGAL - MAY 12: A Christian pilgrim is seen at the Shrine of Our Lady of Fatima as he waits the arrival of Pope Francis in Fatima, Portugal on May 12, 2017. (Photo by Pedro Trindade/Anadolu Agency/Getty Images)

FATIMA, PORTUGAL - MAY 11: A worshipper holds a candle during a night mass in the Sanctuary of Fatima on May 11, 2017 in Fatima, Portugal. Pope Francis will be attending the Sanctuary of Fatima, in Portugal, on May 12 and 13 to canonize two Portuguese shepherds, Jacinta and Francisco Marto. It is said they were witnesses to the apparition of what they believed was the Virgin Mary, together with their aunt Lucia Santos, during the 100 anniversary. Thousands of pilgrims are expected to gather to the centenary celebration. (Photo by Pablo Blazquez Dominguez/Getty Images)

A woman holds a figure of Our Lady of Fatima as she waits for the arrival of Pope Francis at Fatima Sanctuary in Fatima, central Portugal, on May 12, 2017. Two of the three child shepherds who reported apparitions of the Virgin Mary in Fatima, Portugal, one century ago, will be declared saints on May 13, 2017 by Pope Francis. The canonisation of Jacinta and Francisco Marto will take place during the Argentinian pontiff's visit to a Catholic shrine visited by millions of pilgrims every year. / AFP PHOTO / PATRICIA DE MELO MOREIRA (Photo credit should read PATRICIA DE MELO MOREIRA/AFP/Getty Images)

An Angolan woman pays a promise walking on her knees at Fatima Sanctuary in Fatima, central Portugal, on May 11, 2017. Two of the three child shepherds who reported apparitions of the Virgin Mary in Fatima, Portugal, one century ago, will be declared saints on May 13, 2017 by Pope Francis. The canonisation of Jacinta and Francisco Marto will take place during the Argentinian pontiff's visit to a Catholic shrine visited by millions of pilgrims every year. / AFP PHOTO / PATRICIA DE MELO MOREIRA (Photo credit should read PATRICIA DE MELO MOREIRA/AFP/Getty Images)

TOPSHOT - A group of pilgrims arrive at Fatima Sanctuary in Fatima, central Portugal, on May 11, 2017. Two of the three child shepherds who reported apparitions of the Virgin Mary in Fatima, Portugal, one century ago, will be declared saints on May 13, 2017 by Pope Francis. The canonisation of Jacinta and Francisco Marto will take place during the Argentinian pontiff's visit to a Catholic shrine visited by millions of pilgrims every year. / AFP PHOTO / PATRICIA DE MELO MOREIRA (Photo credit should read PATRICIA DE MELO MOREIRA/AFP/Getty Images)

A pilgrim holds a rosary on her hand as she walks on her knees at Fatima Sanctuary in Fatima, central Portugal, on May 11, 2017. Two of the three child shepherds who reported apparitions of the Virgin Mary in Fatima, Portugal, one century ago, will be declared saints on May 13, 2017 by Pope Francis. The canonisation of Jacinta and Francisco Marto will take place during the Argentinian pontiff's visit to a Catholic shrine visited by millions of pilgrims every year. / AFP PHOTO / PATRICIA DE MELO MOREIRA (Photo credit should read PATRICIA DE MELO MOREIRA/AFP/Getty Images)

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For days, pilgrims carrying candles, rosaries and roses have made their way to the statue dedicated to Our Lady of Fatima, with some completing the last part on their knees.

Portugal has boosted security ahead of the pontiff's visit and has reinstated border controls restricting freedom of movement from other European states.

In a video message to the people of Portugal before his journey, Pope Francis said he would "present" himself to the Virgin Mary.

Visions of the Madonna are very important in the Catholic faith and the story of Fatima is universally known among Catholics.

What are the 'Three Secrets of Fatima'?

While grazing their sheep on May 13, 1917, Francisco and Jacinta Marto and their cousin Lucia dos Santos said they saw the first of several visions of the Virgin Mary who had three messages for the world.

Two years later, both of the Martos siblings died of influenza leaving Lucia to share the secret messages with the world.

At first, the children were not believed. But their visions were accepted as authentic apparitions by the Church in 1930.

According to the prophecies written down by Lucia, the first vision the children saw was one of hell and came with a call to prayer and conversion, which if not heeded would lead to another world war.

The second was that Russia would "spread her errors" throughout the world if people did not turn to God.

It wasn't until 2000 that the Vatican disclosed the long-awaited third secret, describing it as a prediction of the 1981 assassination attempt of Pope John Paul II.

John Paul II had credited the Virgin Mary with saving his life and one of the bullets fired at him was placed in the crown of a statue of her in Fatima.

Lucia died in 2005 and is currently on the track for beatification, the first step towards becoming a saint. However, the process could not start until after her death.

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Kids who Catholics believe foretold WWII, Communism to become saints - AOL