Anaheim, California (CNN) In the end, it was the most mild-mannered of the presidential candidates who may have dealt the most searing blow so far to Donald Trump.

In a fascinating twist to the 2016 Republican presidential race, neurosurgeon Dr. Ben Carson essentially threw down the gauntlet Wednesday and asked evangelical Republicans to choose sides by questioning the authenticity of Trump's faith. Speaking to reporters before a large rally here in Anaheim, Carson was asked by a reporter how he was different from Trump.

His answer was short and direct.

"Probably the biggest thing -- I've realized where my success has come from and I don't in anyway deny my faith in God," Carson said.

He explained what he meant by quoting what he said was one of his favorite bible verses.

"By humility and the fear of the Lord are riches and honor and life and that's a very big part of who I am. I don't get that impression with him," Carson said of Trump. "Maybe I'm wrong, but I don't get that."

Carson, who is a Seventh Day Adventist, has made faith a cornerstone of his campaign message. Powered by the backing of evangelicals, Carson is polling highly in Iowa, a state where about half of caucus-goers this winter are expected to be born-again Christians.

Carson's comments seemed to key off some recent queries to Trump about his faith. In one notable exchange at the Family Leadership Summit in Ames, Iowa, earlier this summer, Trump said he had never sought forgiveness from God.

Trump said people had been shocked when they found out that he was protestant. "I am Presbyterian. And I got to church and I love God and I love my church."

But when moderator Frank Luntz asked Trump whether he'd ever asked God for forgiveness, Trump said "I am not sure I have."

"I just go on and try to do a better job from there. I don't think so," Trump said at that forum, in a response that troubled some evangelical Republican voters. "I think if I do something wrong, I think I just try and make it right. I don't bring God into that picture."

Trump also has started to regularly say on the campaign trail that the Bible is his favorite book -- though he declined to share his favorite verse when asked by a reporter last month.

It was an abrupt shift in strategy for Carson, who has impressed voters with his relaxed and sometimes soft-spoken demeanor. As he and Trump have surged in the polls in recent weeks, Carson had complimented Trump's directness; and the real estate magnate had said he was reluctant to attack the neurosurgeon because he was nice guy.

But that changed this week during Carson's trip to California as he moved into the No. 2 spot in the polls. Before questioning Trump's faith on Wednesday, Carson skewered his immigration plans -- noting during a Tuesday appearance at the Commonwealth Club in San Francisco that it would be unfeasible to round up thousands of undocumented workers as Trump has suggested.

Carson doubled down on his immigration critique of Trump on Wednesday in Anaheim before his rally (though he did not mention Trump by name during the main event).

"It's a question that I'm open to discussing with anybody, if anybody can show me how you can actually round up all these people who aren't necessarily going to be cooperative, and how that's not going to jam up the court system and cost enormous amounts of money and disrupt the farming industry and disrupt the hotel industry, and disrupt a lot of service industries and create total chaos. I'm perfectly happy to listen," Carson said.

"I'm not a person who plants my feet on something like that and says you can't possibly have any other alternatives. But I don't see it and I've looked at it very carefully ... What you have to do is look at what works for the entire society and I don't see that as being something that works for the entire society."

Trump responded to Carson by returning the fire on Wednesday evening, tweeting: "Wow, I am ahead of the field with Evangelicals (am so proud of this) and virtually every other group, and Ben Carson just took a swipe at me."

Wow, I am ahead of the field with Evangelicals (am so proud of this) and virtually every other group, and Ben Carson just took a swipe at me — Donald J. Trump (@realDonaldTrump) September 10, 2015

While Trump has held a steady position at the top of the polls, conversations with voters here in Anaheim suggest that his appeal may be waning after weeks of dominating the headlines with his sharp jabs and harsh insults hurled at other candidates and public figures.

Among the thousands of voters who flocked to see Carson in Anaheim were a number who said they were leaning heavily toward the retired pediatric neurosurgeon, but still weighing their options among several of the Republican candidates. Many were quick to rule out Trump, however. Several noted the contrast between Trump's bluster and bravado, and Carson's quiet, thoughtful demeanor.

Amy Casillas, who dashed around the Anaheim Convention Center before Carson's rally trying to get a yard sign to show her support for the neurosurgeon, said she was drawn to Carson's moral values and his authenticity.

Asked if she could support Trump, she answered in a word: "Never."

"He's a very sarcastic man," said Casillas, who described herself as a housewife from Glendale, California, and said she distrusted what she described as Trump's overnight conversion to conservative values.

"It's all a show with him," she said.

Bob Greenspan, a retired lawyer from Mission Viejo, California, said he was originally drawn to Trump because he had brought attention to the immigration issue, said he did not feel comfortable with the idea of Trump in the Oval Office.

"He's got a very limited vocabulary," Greenspan said. "'Amazing,' 'Incredible,' 'I'm the best,' 'I create the most jobs' -- but it's just Trump-speak."

Greenspan recalled how Trump declined to name his favorite verses from the Bible, or even psalms.

"He didn't know any," he said. "He says he's religious, but he refused to cite any Bible verses, or psalms. He shied away from that and changed the subject. As usual."

Hope Aspengren of Murrieta, California, a client advocate at a crisis pregnancy center who attended Carson's rally, said she was fully behind the pediatric neurosurgeon and worried that Trump would be "a loose cannon."

"He could get us into some serious messes. He's not diplomatic at all. It's my way or the highway," she said. "You can't be that way as president."