The religious historian George Marsden once quipped that in the 1950s and 1960s an evangelical Christian was “anyone who likes Billy Graham.” But when Billy Graham was asked to define the term in the late 1980s, he replied, “Actually, that’s a question I’d like to ask somebody too.” As it turned out, even America’s most famous evangelical preacher couldn’t describe what the term meant.

Graham isn’t alone. While the word evangelical pops up in American media to describe everything from mega-churches to voting blocs, few people seem to know what an evangelical is exactly. Those who claim to know often disagree.

The disparate nature of evangelicalism makes its members difficult to define. They don’t have a single authority like the Roman Catholic pope or Mormon First Presidency, so you can’t just phone a central office and ask for the official definition. Since they span a range of denominations, churches, and organizations, there is no single membership statement to delineate identity. As a result, individual observers are left to decide how to define what makes someone or something evangelical. To the pollster, it is a sociological term. To the pastor, it is a denominational or doctrinal term. And to the politician, it is a synonym for a white Christian Republican.

So what is an evangelical, for the love of God, and why does it even matter? The answer requires an understanding of both the history and theology of the movement.

The term evangelical derives from the Greek word euangelion meaning “gospel” or “good news.” Technically speaking, evangelical refers to a person, church, or organization that is committed to the Christian gospel message that Jesus Christ is the savior of humanity. The Greek root word is used in the New Testament and was popularized in the first centuries A.D. to distinguish the love-centric movement of Jesus followers from the violent Roman Empire that often made its own “good news” announcements to celebrate military victories.

But words are more than their etymologies and dictionary definitions. They carry connotations with them too, which change over time and across geographies as they are used in different ways and settings.

According to the Institute for the Study of American Evangelicals at Wheaton College outside of Chicago, Martin Luther first used the Latinized form of the word evangelium to describe the non-Catholic churches birthed by the Protestant Reformation in the 1500s.

But the term largely took hold in the English-speaking world more than a century later during the Great Awakening, a series of revivals in Britain and the American colonies led by fiery preachers such as Jonathan Edwards and George Whitfield. Due to their influence, evangelicalism became a synonym for revivalism, or a fervent expression of Christianity marked by an emphasis on converting outsiders. By the early 1800s, it was “by far the dominant expression of Christianity” in the United States.

In some ways, Christianity took a beating in the early 1900s in America. The carnage of two World Wars and a Great Depression raised questions about whether God existed, and if so, whether God was both powerful and good. And modern science raised doubts about the viability of Christianity’s explanations for the origins of life. Evangelical leaders spanning denominations contemplated forming an organization to represent what one pastor called “the unvoiced multitudes,” and in 1942, the National Association of Evangelicals was born.

While the NAE couldn’t claim to be the sole or definitive voice speaking on evangelicals’ behalf, it helped redefine the term. According to Robert Wuthnow, the director of Princeton University’s Center for the Study of Religion and the author of Inventing American Religion, the de facto definition for evangelical was any person who belonged to a church aligned with the 40-odd denominations under the NAE’s umbrella.

But in 1976, the term went mainstream when a peanut farmer named Jimmy Carter won the Democratic primary and then the general election. He became the first U.S. President to call himself a “born again” evangelical Christian. Pundits scrambled to understand who evangelicals were and how many existed. Newsweek ran a cover story declaring 1976 the “year of the evangelical.”

Not to be left out, more conservative evangelicals who diverged from Carter politically began mobilizing under new organizational banners like the Christian Coalition and the Moral Majority—collectively labeled the religious right. These politically active conservative Christians were well-funded and media savvy, but they were only able to become synonymous with evangelicalism with the help of American pollsters.

When the NAE was founded, Wuthnow says, new reports estimated the organization represented about two million people. Ten years later, in 1953, the NAE claimed it represented 10 million. Lack of data made the number impossible to verify at the time. In 1967, one New York Times reporter estimated the number to be around 20 million. With Carter’s emergence, George Gallup decided to conduct a poll in which he defined evangelical as anyone who claimed to be “born again.” This simplistic definition led Gallup to report that as many as 50 million Americans—a third of the eligible electorate—were evangelical.

The “unvoiced multitudes” now had their microphone. Conservative Christian leaders embraced the spotlight and their newfound legitimacy, and pastors began showing up on the nightly news and in major magazines to endorse candidates and push forward policy proposals. Over time and in the minds of many, evangelical became a catch-all term for politically conservative Christians.

In recent years, however, a range of definitions for evangelical have been proposed. Many polling firms, like Pew Research, count everyone who identifies themselves as “evangelical” or “born again.” A prominent Christian polling firm, Barna Group, has traditionally used a very specific nine-question definition that requires, for example, the person to claim they believe Satan exists. And Molly Worthen, University of North Carolina professor and author of Apostles of Reason: The Crisis of Authority in American Evangelicalism, defines evangelicals as Christians who are wrestling with a set of specific questions.

This may all sound obscure, but it matters.

“Evangelicals’ influence may be waning, but it is still important for us to understand who evangelicals are, who speaks for them, and what the future looks like,” says David Kinnaman, the president of Barna Group and author of the forthcoming Good Faith: Being a Christian When Society Thinks You’re Irrelevant and Extreme. “The way one defines ‘evangelical’ influences the story they tell about the most influential group within the most influential religion in the world’s most influential country.”

Depending on how you define the term, evangelicals comprise between 7 percent and 47 percent of the American population. Divergent definitions have led to inconsistent, even contradictory survey results about evangelicals’ beliefs and characteristics. Reports based on these surveys can shape elections, public policies, and broader public opinion.

The most widely accepted definition of evangelical is probably the one put forward by historian David Bebbington in 1989. It’s called the “Bebbington quadrilateral” because it identifies evangelicals as Christians who share four main qualities:

  • Biblicism: a high regard for the Bible
  • Crucicentrism: a focus on Jesus’s crucifixion and its saving effects
  • Conversionism: a belief that humans need to be converted
  • Activism: the belief that faith should influence one’s public life

While some have criticized this quartet of beliefs as being too capacious, many scholars—including sociologists like Wuthnow and historians like Marsden—believe it is the best of all proposed definitions. LifeWay Research and the National Association of Evangelicals have recently suggested a definition that closely mirrors Bebbington’s after receiving input from a diverse group of religious thinkers and academics. Even Barna’s David Kinnaman says his firm has been experimenting with a similar definition in some overseas studies in place of their nine-point criteria.

This four-point criterion is the most widely accepted definition among scholars and the only one approved by the NAE, America’s largest coalition of evangelicals. And it provides a tool to help researchers classify evangelicals by belief, which seems like an appropriate way to handle almost any religious group. Additionally, it leads to a more reasonable estimate of the movement’s size—somewhere around a quarter of all Americans.

Like all definitions, Bebbington’s description of evangelical isn’t perfect. But it allows the term to be appropriately narrow while making room for a diverse range of Christians who fit the bill. If it sounds complex, it should be. Religion is always more complicated than “hallelujah” and “amen.”