The 100th anniversary of the first Marian Apparition in Fatima, Portugal, is this week on May 13. But what exactly are the so-called secrets of Fatima?

Three shepherd children, 100 years ago, claimed to have seen supernatural appearances of the Virgin Mary, the mother of Jesus. Before Mary appeared, one of the children — Lucia — saw an angel.

Our Lady warned that if people did not stop offending God, He would punish the world.

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“In the summer of 1916, Lucia dos Santos, the youngest of a family of seven children, began shepherding her family’s flock along with three of her friends, Teresa Matias and her sister Maria Rosa, and Maria Justino,” as Catholic News Agency reported. “During this time, an angelic figure appeared before the girls three different times as they were praying the rosary in the fields, but did not speak to them.”

The year after that, the Virgin Mary appeared to Lucia Santos, and to her cousins Francisco and Jacinta Marto, six separate times between May 13, 1917, and October 13, 1917. Along with her apparitions, Mary gave the children three secrets.

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“At that moment the First World War raged in Europe, introducing mankind to the most savage forms of warfare seen to that date. In far-off Moscow, Lenin prepared the revolution that would overturn the Russian social order in November 1917 and eventually engulf almost half the peoples of the earth,” EWTN.com reported. “It was in that context that heaven condescending to earth to provide the antidote to the moral and social evils of the world — the message of Fatima.”

Lucia revealed the first two secrets in her memoirs. Pope John Paul II published the third secret in 2000.

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“On July 13, Mary revealed what has been come to be known as the ‘Great Secret’ of Fatima, a secret that Lucia divided into three parts and slowly revealed to the public over time. Two parts of the secret were revealed in 1941, when Lucia was asked to record her memoirs by the local bishop. The rest was not revealed until the year 2000, per Mary’s instructions, initially, and then later instructions of the Holy See,” according to Catholic News Agency.

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“According to the interpretation offered by the doctrinal congregation, the three secrets involve hell, World War I and World War II, and the 1981 Pope John Paul II assassination attempt,” as Cruxnow.com reported.

The three secrets given to the children were these:

1.) A vision of hell. “The rays [of light] appeared to penetrate the earth, and we saw, as it were, a vast sea of fire,” Lucia wrote in her memoirs. “Plunged in this fire, we saw the demons and the souls [of the damned]. The latter were like transparent burning embers, all blackened or burnished bronze, having human forms. They were floating about in that conflagration, now raised into the air by the flames which issued from within themselves, together with great clouds of smoke. Now they fell back on every side like sparks in huge fires, without weight or equilibrium, amid shrieks and groans of pain and despair, which horrified us and made us tremble with fright (it must have been this sight which caused me to cry out, as people say they heard me).”

Lucia lived to be 97. She died in 2005.

2.) That World War I would end, but another war would take place if Russia did not convert. “In the apparition of July 13, Our Lady warned the three seers that if people did not stop offending God, He would punish the world ‘by means of war, hunger and persecution of the Church and of the Holy Father,’ using Russia as His chosen instrument of chastisement,” according to Fatima.org. “She told the children that ‘to prevent this, I shall come to ask for the Consecration of Russia to My Immaculate Heart’ and promised that, by this single public act, Russia would be converted and peace would be given to the world.”

3.) The persecution of Christians. “As it was revealed, the third secret talks about a ‘bishop dressed in White,’ which Lucia and the two others believed was the pope, ‘killed by a group of soldiers who fired bullets and arrows at him,’ killing also bishops, priests, men and women religious and lay people,” according to Cruxnow.com.

Francisco and Jacinto died young — while Lucia lived to be 97. She died in 2005.

Pope Francis is scheduled to visit Fatima this weekend, where he will canonize Francisco and Jacinta as saints.