U.S. Republican presidential candidate Dr. Ben Carson speaks at the North Texas Presidential Forum hosted by the Faith & Freedom Coalition and Prestonwood Baptist Church in Plano, Texas, Oct. 18, 2015. Reuters

Retired neurosurgeon Ben Carson has long embraced his Christian beliefs, bringing prayer into the operating room and the campaign trail. He was twice baptized under the Seventh-day Adventist faith, which he describes in his book "Gifted Hands."

His religion has become part of his presidential campaign after Republican front-runner Donald Trump attacked Carson's faith as he discussed Carson's polling numbers in the important early caucus state Iowa during a campaign rally in Florida over the weekend. "I love Iowa. And, look, I don't have to say it, I'm Presbyterian," Trump said. "Can you believe it? Nobody believes I'm Presbyterian. I'm Presbyterian. I'm Presbyterian. I'm Presbyterian. Boy, that's down the middle of the road, folks, in all fairness. I mean, Seventh-day Adventist, I don't know about. I just don't know about."

Unlike most Christians, Seventh-day Adventists observe the Sabbath Saturday. The church embraces a literal view of the Bible. "We believe that the biblical events recorded in Genesis 1-11, including the special creation of human beings, are historical and recent, that the seven days of creation were literal 24-hour days forming a literal week, and that the Flood was global in nature," reads the church's official website. Carson has said he accepts the church's teachings, saying he was "proud of the fact that I believe what God has said … that I believe in a literal six-day creation."

The church's founder, William Miller, was a Baptist preacher from Upstate New York who claimed Jesus Christ would return to earth Oct. 22, 1844. The Seventh-day Adventist Church now has more than 18 million members globally and 1.2 million in North America. Followers claim they are waiting for the Second Coming and the end of the world.

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David Holland, a professor at the Harvard Divinity School, told CBS News that some Christians are wary toward Seventh-day Adventism but generally accept the church. "Theologically, Seventh-day Adventism tends to be a bit closer to mainstream Christianity," Holland said.

Carson has said his faith has guided him in the past. Before surgery, he said he would ask for God’s help. “Lord, you be the neurosurgeon,” he has described himself thinking. “I’ll be the hands.”

He has said that he is open to other faiths that worship God. “I spend just as much time in non-Seventh-day Adventist churches because I’m not convinced that the denomination is the most important thing,” he said during a 1999 interview with the Religion News Service. "I think it’s the relationship with God that’s most important."