The Third Secret, which some thought foretold the exact date of doomsday or lurid church scandals, actually warned of the assassination attempt against the Pope, the Vatican said.
The so-called secrets of Fatima came from three shepherd children who saw visions of the Virgin Mary in a Portuguese field in 1917.
The Pope visited Fatima Saturday to beatify two of the children and finally released the long-awaited third secret.
Vatican Secretary of State Cardinal Angelo Sodano told a crowd of believers that the third revelation describes "a bishop clothed in white" who "falls to the ground, apparently dead, under a burst of gunfire."
The Catholic Church interprets the vision as a description of the pope's near-fatal shooting in 1981. A Turkish gunman shot Pope John Paul II in St. Peter's Square on May 13, 1981, the anniversary of the first reported vision at Fatima.
John Paul II has credited the Virgin Mary with saving his life, and one of the gunman's bullets was placed in the crown of the Madonna statue in Fatima.
An Australian man even hijacked a plane in 1981 to try to force the Vatican to reveal the Third Secret.
Sister Lucia, a 93-year-old nun who is the last survivor of the three children, first told church officials about the first two secrets in 1943. The third was written down and sent in a sealed envelope to the Vatican.
Church officials said they were still holding back part of the third secret, but cautioned it contained no earth-shattering announcements.
"The events to which the third part of the Secret of Fatima refers now seem part of the past," said Sodano, who added the complete text of the secret will be released after the Vatican writes a thorough explanation of the prophesy.
On Saturday morning, more than 800,000 pilgrims attended a ceremony during which the pope beatified Francisco Marto and his sister Jacinta, who, with surviving visionary Sister Lucia Dos Santos, claimed to have been passed three secrets in a series of visions of the Virgin Mary in 1917.
At the ceremony, Sodano said the pope had asked the church hierarchy to make public the third secret of Fatima, which, Vatican spokesman Joaquin Navarro later explained, foresaw a 1981 assassination attempt on the pontiff.
The first two Fatima secrets--referring to God's punishment for mankind's sins, the need for repentance, and a prophesy of Russia's attempt to wipe out religion under communism--had long been known, and expectations the third would be revealed had been running high.
Sodano said the text of the secret constituted a prophetic vision comparable to that of the Holy Scriptures.
"The Fatima vision concerns above all the struggle of atheist systems against the Church and against Christians. It describes the immense suffering of witnesses of faith in the last century of the second millennium," Sodano said in a statement released here.
"According to the interpretation of the young shepherds, the bishop dressed in white who prays for all the faithful is the pope. He, too, walking painfully towards the cross among the bodies of martyrs (bishops, priests, believers and non-believers), falls to the ground as if dead, hit by gunshots," Sodano said.
A Turkish gunman, Ali Agca, carried out an assassination attempt on the pope in Rome's Saint Peter's Square 19 years ago to the day.
"It is clear today that the bishop dressed in white was Pope John Paul II," Vatican spokesman Navarro said.
The pope himself referred to the attack during the ceremony, saying: "I want once more to celebrate the bounty of the Lord towards me, when, struck a grievous blow on May 13, 1981, I was saved from death."
He has long credited the Virgin Mary with his survival.
The third Fatima secret had previously been known only to the surviving visionary, 93-year old Sister Lucia Dos Santos, the pope, and German Cardinal Joseph Ratzinger, the head of the Vatican Congregation of the Doctrine of the Faith.
The pope met Sister Lucia, a cloistered nun, before the ceremony.
The ceremony for the beatification of Francisco Marto and his sister Jacinta took place on the day that marked the anniversary of their first vision.
Both died of flu within three years of their reported visions.
Throughout his homily, the pope stressed the humility of the two shepherds, who came from a poor family and were virtually illiterate.
They are the first children to be beatified, the penultimate step before sainthood.
