He is running on his faith and not his religion. There is a difference. I have looked at the beliefs of the Seventh Day Adventist and there is a bigger difference between Catholicism and Presbyterian than Adventist and Presbyterian.
His faith and religion are intertwined. Ask him about Ellen White. SDA is arguably a modern borderline cult with Christian strains, like Mormonism and JW but closer to Orthodoxy than either of those. If one can vote for Romney, one can vote for Carson. Presbyterianism is a bona fide Protestant denomination that came out of the reformation. As such, it has much more in common with Orthodoxy than the SDA.