On the campaign trail, South Bend Mayor Peter Buttigieg has actively highlighted his own religious faith, and called for a resurgence of a “religious left” in the United States.

“The left is rightly committed to a separation of church and state… but we need to not be afraid to invoke arguments that are convincing on why Christian faith is going to point you in a progressive direction,” Buttigieg told USA Today in a recent conversation about religion and faith, characterizing conservative Christians as “saying so much about what Christ said so little about, and so little about what he said so much about.”

But an increasing number of right-wing figures, displeased that Buttigieg has taken to pointing out the plank in their eye, have begun undercutting Buttigieg’s religiosity by dismissing him as not “really” Christian—not because he’s gay, but because, like 11 American presidents, he’s an Episcopalian.

“He says he’s a traditional Episcopalian, whatever that means these days,” Fox News host Laura Ingraham said in a segment that dismissed Buttigieg as “but another creation of a media apparatus desperate to oust Trump.”

“If Buttigieg thinks evangelicals should be supporting him instead of Trump, he fundamentally does not understand the roots of Christianity,” conservative commentator and reformed #NeverTrump Republican Erick Erickson tweeted on Sunday. “But then he is an Episcopalian, so he might not actually understand Christianity more than superficially.”

After online Episcopalians and their defenders chased Erickson’s tennis ball into traffic, the commentator—who previously accused Buttigieg of thinking “Jesus would be okay with beastiality [sic]”—has since mounted a days-long crusade of tweets and blog posts on his website The Resurgent deriding the Episcopal Church, its leadership and laity.

Buttigieg, Erickson said in one post, “is not really Christian so much as he is Episcopalian.”

As evidence that the Episcopal Church is no longer, in his view, a Christian one, Erickson pointed to some Episcopal leaders supporting access to abortion and the openness of some Episcopal priests to questions about the physical resurrection of Jesus, which Erickson considers heretical.

“The Episcopal Church itself is no longer a Christian institution and those who remain will either walk into a Christian denomination or walk into atheism potentially with a helping of spirituality,” Erickson wrote in a blog post on Tuesday, prompted by a request from The Daily Beast for him to elaborate on his feelings regarding the Episcopal Church.

“The people remain behind [sic] are a bunch of rich, white people who want to feel good about themselves by feeling morally superior to their neighbors who really believe that whole ‘Jesus died for me’ stuff,” Erickson said.

Among the rich, white people sitting in Episcopal pews: President Donald Trump, whose third marriage was held at Bethesda-by-the-Sea, an Episcopal church in Palm Beach where the first family often attends Christmas and Easter services. (Trump identifies as a Presbyterian, although he’s not particularly doctrinal.)

As a religiously devout son of the American heartland and Afghanistan veteran, Buttigieg has complicated traditional right-wing narratives about Democratic presidential hopefuls as effete coastal elitists. But as his fundraising and poll numbers have risen, so too has negative coverage from the right. Opposition researchers and political analysts have told The Daily Beast that attacks on Buttigieg’s sexuality would almost certainly backfire, but long-simmering doctrinal and cultural differences between mainline Protestants and evangelical Christians might be an easier fault line to exploit.

“In our time, the Episcopal Church is a generally progressive denomination that ordains gay and lesbian bishops, makes room for liberation theology, was never part of the Christian Right, and lost the conservative congregations that hated these developments,” Prof. Gary Dorrien, who teaches social ethics, theology, and philosophy of religion at Union Theological Seminary, told The Daily Beast.

“The history of that division is a theological,” added the Rev. Dr. Kelly Brown Douglas, dean of Episcopal Divinity School at Union Theological Seminary. “The Episcopal Church faith tradition has been concerned with salvation as it talks about issues of social justice and issues of freedom—a movement toward ‘God’s Just Earth,’ so that the issues that really become central to our concern are issues of how one treats those who have been marginalized.”

That espousal of the so-called “Social Gospel,” Dr. Douglas said, generally stops short of more doctrinaire views of social issues like abortion and same-sex relations—“we leave people the ability to makes choices... which in evangelical and Catholic faith traditions, they don’t generally do”—has left evangelical Christians skeptical of the legitimacy of the Episcopal Church.

“Sometimes righteousness becomes self-righteousness,” Dr. Douglas said. “These things divide us theologically, and so I think that’s what you see, is an historical theological divide, because of the way we understand the centrality of the Gospel message.”

But beyond those long-held philosophical and religious differences, Dr. Douglas said, Buttigieg’s status as a gay Christian is likely inseparable from the critiques against the Episcopal Church at large.

“The very fact that Mayor Pete is a gay man and he professes to be Episcopalian, well, that would be the rub for evangelical Christians, because fundamentalists—and not all evangelicals, to be clear: fundamental evangelicals, the right-wing branch—they are not going to accept someone that claims to be Christian and also gay,” Dr. Douglas said. “We need to be clear as to what the rub is in this regard.”

For Buttigieg, whose personal distaste for religious hypocrisy runs deep, attacks on his faith are almost definitionally unworthy of direct engagement.

“I think good faith is so important,” Buttigieg told The Daily Beast last month in a conversation about his relationship with Vice President Mike Pence. “Even when I have a very stark disagreement with somebody, it’s just a lot easier on both sides I think for us to navigate it if we both understand where the other is coming from, and believe that those different opinions are something we came by honestly.”

Accordingly, Buttigieg’s campaign told The Daily Beast that Erickson and Ingraham’s broadsides against the Episcopal Church don’t merit engagement.

“We’ll let whatever that meltdown was speak for itself,” said Lis Smith, Buttigieg’s communications director. “The mayor has spoken extensively about his faith and how it guides him as a human being and elected official. It’s not surprising that this administration’s defenders are sensitive about being called out for their hypocrisy.”