The Sistine Chapel is a place full of miracles - from the story of Creation and the Parting of the Red Sea to the Last Judgment.

As the cardinals solemnly march into the chapel Tuesday to begin the process of choosing a new pope, they will sing "The Litany of the Saints," naming the saints one by one and asking for their prayers.

To be considered for sainthood, a person has to have performed two miracles. In Catholic theology, miracles are the proof of sainthood. These miracles are considered acts of divine intervention, friendly help from God above.

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At the Vatican, the term "miracle" is not taken lightly.

Across from St. Peter's is the Congregation for the Causes of Saints. Behind its huge doors, investigators spent years sifting through reports of miracles, trying to determine whether they are truly miraculous.

"It has to be lasting, instantaneous, perfect and scientifically inexplicable," said Monsignor Robert Sarno, whose job is to verify miracles and show that a person is worthy of sainthood.

"When the church canonizes someone, it's not like giving a posthumous medal to somebody," Sarno said. "It's not like saying, 'Hey, you did a great job.'"

Of the 265 popes, 78 are saints.

Sixteen, including Blessed Pope John Paul II, are halfway there. The Vatican has confirmed one miracle attributed to him. He cured a French nun, Sister Marie Simon-Pierre, of Parkinson's disease after she prayed to him.

He will be sainted if the church can verify a second miracle.

"The idea is the miracle shows that you're definitively in heaven and it's a good sign of God's favor," said Father James Martin of the Society of Jesus. "And Christians believe that you're praying to someone who is basically putting in a good word for you."

ABC News' David Wright contributed to this story.