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HARRISBURG, PA. - They called it the church riot.

Sen. John McCain, who is known for his reticence invoking faith on the campaign trail, was once dubbed a "Hell's Angel" for rioting against his captors in Vietnam in order to hold Sunday church services.

It is a story unknown by a public still getting to know McCain and searching for shared values with the candidates. This Saturday, McCain and Sen. Barack Obama will separately answer questions from evangelical leader Rick Warren at his Saddleback Church in Southern California.

In an interview, McCain talked about how his faith was tested during his years as a prisoner of war from 1967 to 1973, and reminisced about his appointment as informal chaplain to his cellmates.

"There were many times I didn't pray for another day and I didn't pray for another hour -- I prayed for another minute to keep going," said McCain, who was brought up Episcopalian but now worships at the North Phoenix Baptist Church, a Southern Baptist church.

McCain said his faith in God informs his decisions on issues of public policy. Christian conservatives are skeptical of McCain's commitment to many of the issues they care about such as abortion and marriage. They have also been disappointed in his embrace of embryonic stem cell research. But McCain said he wrestled with that decision and hopes technology soon renders it obsolete.

In Vietnam, McCain's fellow prisoners say their faith was a matter of life and death. "We knew we had to have some belief greater than ourselves," said Orson Swindle, a Marine captain who spent six years in captivity.

The prisoners had developed a tap code system for communicating through the walls. Through that tapping, the prisoners decided that every Sunday, after they'd eaten their rice, the highest-ranking officer would cough and say the letter 'c' for church. The prisoners would then recite the Pledge of Allegiance, the Lord's Prayer and the 23rd Psalm, in plural: "Yea, though we walk through the valley of the shadow of death we will fear no evil."

McCain's friends say they believe God had a plan for him. He, too, acknowledges that idea, though cautiously.

"I can't help but feel like that to some extent, and I'm not a fatalist," said McCain. "I do think we make our own choices, but certainly I think I was meant to serve a cause greater than my self interest."