Republican presidential candidate Donald Trump, speaking on the USS Iowa in Los Angeles in September. (Lucy Nicholson/Reuters)

JACKSONVILLE, Fla. -- Donald Trump on Saturday contrasted his Presbyterian faith with that of Seventh-day Adventists, apparently attempting to draw attention to and raise doubts about Republican presidential rival Ben Carson's religious beliefs.

Speaking at a campaign rally here, Trump was in the midst of discussing his standing in the polls, including recent Iowa surveys that showed him falling behind Carson in the GOP race. Then, he brought up religion.

"I love Iowa. And, look, I don't have to say it, I'm Presbyterian," said Trump. "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."

Politically, the matter could become an issue in the lead-up to the Iowa caucuses. As the Des Moines Register notes, "some conservatives have argued Seventh-day Adventists … aren't Christians."

Carson is a twice-baptized Seventh-day Adventist, which he describes in his book "Gifted Hands." Seventh-day Adventists are Protestants who observe the Sabbath on Saturday, unlike most Christians, including Presbyterians, who observe it on Sunday. The church also takes a very 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 Web site.

Trump's remark came as he and Carson, two first-time candidates, have risen to the top of the GOP pack. Trump repeatedly criticized Carson during a two-day campaign swing in Florida, calling the soft-spoken retired neurosurgeon "super low-energy" on Friday night.

Asked whether Trump meant to single out Carson's religion with his Saturday comments, Trump spokeswoman Hope Hicks said: "I think the remark speaks for itself."

It's not the first instance of religious tension between Trump and Carson. In September, Carson suggested that Trump doesn’t appear to live out his faith -- a comment he later said he regretted.

Carson's campaign did not immediately respond to a request for comment.