Republican presidential candidate Donald Trump holds up his Bible during a Dec. 29 campaign stop in Council Bluffs, Iowa. (Nati Harnik/AP)

On the lengthy checklist Donald Trump carries in his pocket titled, "FEUDS TO GET INTO," he's finally gotten the chance to cross off one of the big ones. Donald Trump is now in a war of words with Pope Francis, the leader of the Catholic Church and a man considered by millions to be the conduit for the word of God on Earth.

Not that such things would give Trump pause.

Francis conducted Mass at the border this week, intentionally drawing attention to the moral aspect of the fight over immigration.

While flying home from his visit to Mexico, the pope referred obliquely to the Republican presidential front-runner when asked about the businessman's views on immigration.

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said. "This is not in the Gospel." Francis hadn't heard about Trump's proposals about the border, but when informed of them issued a verdict. "I'd just say that this man is not Christian if he said it this way," he said.

Trump responded quickly in a statement on his website. "If and when the Vatican is attacked by ISIS ... I can promise you that the Pope would have only wished and prayed that Donald Trump would have been president because this would not have happened," the statement read. Trump also blamed Mexico for feed the pontiff bad information, because the country "understand[s] I am totally wise to them."

After Pope Francis suggested Republican presidential candidate Donald Trump is "not Christian," Trump told a rally in Kiawah Island, S.C. that the Mexican government manipulated the pope, calling the religious leader's statement "disgraceful." (Reuters)

"For a religious leader to question a person’s faith is disgraceful," Trump said -- but Mexico is worse. "They are using the pope as a pawn and they should be ashamed of themselves for doing so."

This would seem like an odd fight for a politician to pick in any year besides 2016 and with any politician besides Donald Trump.

Since he became pope, Francis has been frustrating American conservatives. Last July, as Francis prepared to come to the United States, Gallup found that approval of him among Americans had fallen -- thanks in part to a steep decline in support among American conservatives. In 2014, 72 percent of conservatives supported Francis. Last year, that number dropped to 45 percent.

In part, that's thanks to Francis's willingness to opine on issues central to American politics. A month before that Gallup poll, the pope released an encyclical demanding action on climate change, a topic that is among the most politically polarized in the country. The pope took the Democrats' side, if you will, and that frustrated a lot of conservatives. On a range of other issues, Francis has shown more flexibility than some Catholics are comfortable with.

Francis's comments about immigration are generally in line with American Catholics. In a November Post/ABC News poll, only 37 percent of Catholics supported deporting all of the undocumented immigrants in the country -- a number closer to the 31 percent of Democrats who supported the idea than the 56 percent of Republicans who did. In January, another Post/ABC poll found that 60 percent of Catholics think that immigrants strengthen the country. That's compared to 65 percent of Democrats and 37 percent of Republicans that said the same thing.

But Trump isn't a Catholic, as he noted when he was asked about Pope Francis's comments disparaging capitalism prior to the pope's visit. In response, Trump trotted out the same argument about the Islamic State -- or as he referred to it, ISIS.

How would you reply, CNN's Chris Cuomo asked, if the pope confronted you about the evils of capitalism? "I'd say, 'ISIS wants to get you,'" Trump replied. "He talks to you about capitalism, and you scare the pope?," Cuomo replied. "I scare the pope," Trump said.

A Vatican spokesman responded to questions about the Islamic State targeting the Vatican. "There is nothing serious to this," he said. "There is no particular concern in the Vatican. This news has no foundation."

On Twitter on Thursday, a Trump adviser suggested one reason the Vatican might not be worried: It has walls.

It's important to note that while the Republican electorate is heavily religious, it is also heavily Protestant, not Catholic. The largest religious voting bloc in Iowa and in many southern states is white evangelical protestants -- a group that's historically often antagonistic to the Catholic Church. (After conservatives started to grow wary of Francis, some evangelical leaders began trying to peel them away.) According to Pew Research data from 2012, 43 percent of Catholics are Republican, versus 48 percent who are Democrats (or who lean to one party or the other). By contrast, 70 percent of evangelicals are Republican. Evangelicals make up a quarter of the population, while Catholics are only a fifth.

Republicans don't really see Trump as particularly religious anyway. Among all adults, only 5 percent told Pew that they thought he was very religious when asked last year. His stumble while speaking to a religious college, referring to a book of the Bible as "Two Corinthians" instead of the more common "Second Corinthians" inspired mostly shrugs. Previously, Trump had declined to name his favorite Bible verse.

As with so many of Trump's other feuds, then, the smart money is that this won't have much of a negative effect on his political prospects in the Republican nomination contest. The Republican Party is more heavily evangelical than Catholic, and conservatives had already grown skeptical of Francis's politics. And, of course, people like Trump getting in fights and defending his positions.

The audience for Trump's comments was probably those voters anyway. When he was talking to Cuomo last summer, Trump indicated that he didn't think the pope would be scared by him anyway. "The pope, I hope," he said, "can only be scared by God."