Tomorrow, when the cardinals walk toward the secret conclave, they'll do so praying for the saints who left behind miracles. But exactly what kind of miracles? Abc's david wright tells us. Reporter:... See More
Tomorrow, when the cardinals walk toward the secret conclave, they'll do so praying for the saints who left behind miracles. But exactly what kind of miracles? Abc's david wright tells us. Reporter: From the miracle of creation, to the parting of the red sea, to the last judgment, the sistine chapel is a room full of miracles. And here at the vatican, the term "miracle" is not taken lightly. Right across from st. Peter's is the congregation for the causes of saints. Behind these huge doors, investigators spend years poring over reports of miracles, trying to determine if they are truly miraculous. It has to be lasting, instantaneous, perfect and scientifically inexplicable. Reporter: In catholic theology, miracles are the proof of sainthood. Acts of divine intervention. Friendly help from above. The idea is the miracle shows that you're definitely in heaven. And it's a sign of god's favor. And christians believe that you're praying to someone who is basically putting in a good word for you. Reporter:78 of the 265 popes are saints. 16 more are halfway there. Including blessed pope john paul. The vatican has confirmed one miracle attributed to him -- curing a french nun afflicted with parkinson's disease. Reporter: Tomorrow as they solemnly march into the sistine chapel - ♪ Reporter: Naming the saints, one by one, and asking for their prayers. miracles, but as they make this momentous decision, looking for friendly help from above. David wright, abc news, rome.
This transcript has been automatically generated and may not be 100% accurate.