Donald Trump holds up his Bible during a campaign stop in Council Bluffs, Iowa, on Dec. 29, 2015. (Nati Harnik/Associated Press)

Sometimes, Donald Trump jokes. He jokes a lot, sometimes because he is actually joking, and sometimes he is joking retroactively in an effort to wave away something he said that annoyed people.

On Wednesday afternoon, it seems to have been a bit of both. When Trump asked audience members in Council Bluffs, Iowa, to raise their hands if they weren't "Christian conservatives" and then asked everyone else — the vast majority — if the non-Christian conservatives should be allowed to stay at the event, it was a goof. Just fun. No big.

Not everyone took it that way. The Council on American-Islamic Relations released a statement citing Trump's "history of targeting religious and ethnic minorities" as reason for the comment to be considered critically. It's a fair point. Coming from another candidate at another time, it probably would be considered an awkward joke. Coming from Trump — after months of suggestions that, for example, Muslims should be barred from entering the United States, added to a federal database and surveilled at their places of worship — the tone and implications are different, especially for someone seeking the presidency.

The politics of the comment are one thing. Iowa is mostly Christian, if not mostly conservative. (In the Iowa caucuses this year, 85 percent of Republicans identified as conservative — but that's just half of the fairly low turnout.) According to data from the Pew Research Center, 77 percent of the state is Christian, with 28 percent identifying as evangelical, the denomination most regularly associated with political Christian conservatism. Trump was clearly in front of a friendly crowd for the question, but that Christian conservative group is still a minority of the state. Trump repeatedly has had trouble figuring out how to expand his political base, and joking about kicking out those who aren't Christian conservatives doesn't seem like the sort of thing that might appeal to non-Christian conservatives.

The implications of a potential president joking about isolating and excluding a minority religious group are another thing entirely. I spoke with Chip Lupu, professor emeritus at George Washington University Law School and author of "Secular Government, Religious People," to get his sense of how Trump's comments might be viewed through the lens of the separation between the government and religious practice.

There are two important flags Lupu identified that would push the specific action Trump took over the line into constitutionally questionable. The first would be if Trump were a representative of or an official with the federal government. But even then, he categorized the question as more unwise than risky.

"Let's pretend it was the president of the United States, in office, speaking in his official position," Lupu said, "who did something as sort of divisive, clumsy and stupid as that. No one would say there was something unconstitutional about it." Were Trump as president raising the question in a "coercive setting" -- say, if he were addressing members of the military — the situation shifts. It's all a bit gray, but that's the combination Lupu identified: government actor seeking explicitly or implicitly to be acting based on religious qualifications.

The bigger question, beyond that specific action, is what a differentiation of religious groups might mean in other ways once Trump took the oath of office. Let's say, for example, that Trump wanted to ensure that only Christian conservatives were appointed to his Cabinet or to the judiciary. (We know already that he has questions about the impartiality of judges of Mexican heritage.) No president can explicitly state that they intend to give one religious group preference over another for a federal position thanks to the prohibition in Article VI of the Constitution that bars the use of a "religious test" as qualification. But, Lupu noted, Trump (or anyone else) could simply appoint only Christian conservatives to those positions, and it would be tricky to suss out the intentionality of it. It's like saying that someone is lying: Are they seeking to mislead you or are they just making a mistake?

Our system is designed to prevent the president from having too much power to act on his own, limiting the number of ways in which any president can influence or target particular groups. (You remember this from fifth grade. Checks and balances.) An example: Trump's proposal to surveil mosques would be carried out by the FBI, presumably, and those affected by it could petition the judicial branch to curtail the process (probably successfully). Other proposals with different aims probably would need to go through Congress, and even then be reviewed by the Supreme Court.

Trump has proposed one idea that would shift the relationship between the religious community and the government. He has repeatedly called for the repeal of the Johnson Amendment, a policy that prohibits 501(c) (3) nonprofits from expressly advocating for the election or defeat of a political candidate. (It's not clear how Trump seized on this issue, but it seems likely that it was raised by evangelical leaders and was seen by Trump as a fairly simple way to demonstrate his support for their concerns.) The reason the barrier exists is because contributions to nonprofits are tax deductible, Lupu said. If you give a church $20,000 that the church spends on a political campaign, you're paying less in taxes because of that giving. That gives churches an advantage in fundraising — and, therefore, in their ability to influence politics.

What's more, he added, "no one talks about getting rid of this restriction for all charities, but only about houses of worship. If only houses of worship got this break where they could spend money on politics and people could still deduct the donations, you would be advantaging religious voices over other voices in the political process."

"I think that would be an unconstitutional preference of a religion," he added, a violation of the establishment clause codified in the First Amendment.

But let's reel this all back in. The prompt for this line of questioning was an off-handed comment from a candidate who prides himself on such things. It was a simple joke about whether non-Christians or nonconservatives (it's not clear which) should be allowed to stay and hear Trump speak. Nothing more, nothing less. And because he's not president, the constitutional ramifications aren't applicable.

For those inclined to be cautious about Trump's familiarity with the Constitution and understanding of the powers of the presidency, though, other questions naturally followed. Maybe in his joke, as the corny old saying goes, was a kernel of truth.