An upcoming book claims that Donald Trump slammed evangelical Christians before the Iowa caucus in 2016, the Guardian reports.

In The Kingdom, the Power, and the Glory: American Evangelicals in an Age of Extremism, Atlantic staff writer Tim Alberta claims that Trump referred to evangelicals as “real pieces of shit.”

Trump, Alberta writes, was stung by mockery he received after speaking at Liberty University, the Christian school in Virginia, where he shared his favorite Bible verse, from the Second Epistle to the Corinthians. Trump called the verse to “2 Corinthians,” the written attribution, and not “Second Corinthians,” as it is usually called colloquially.

Supporters of Ted Cruz, a primary rival to Trump at the time, cited the incident as proof that Trump wasn’t really a committed Christian. Trump reportedly told a Republican official in Iowa, “You know, these so-called Christians hanging around with Ted are some real pieces of shit.”

Trump remains mostly popular with many Christians. According to a recent report from the Public Religion Research Institute, 61% of white evangelical Protestants hold a favorable view of the former president.

Alberta’s book, about divisions among American evangelicals, was called “sometimes overly personal yet well researched and comprehensive” by a critic for Kirkus. It is slated for publication by Harper/HarperCollins on Dec. 5.

Michael Schaub is a contributing writer.