The mainline Protestant churches (also called mainstream American Protestant and oldline Protestant) are a group of Protestant churches in the United States that contrast in history and practice with evangelical, fundamentalist, and charismatic Protestant denominations. Mainline Protestants were a majority of all Christians in the United States until the mid-20th century, but now constitute a minority among Protestants. Mainline churches include the United Methodist Church (UMC), the Evangelical Lutheran Church in America (ELCA), the Presbyterian Church (U.S.A.) (PCUSA), the Episcopal Church, the American Baptist Churches, the United Church of Christ (Congregationalist), the Disciples of Christ, Reformed Church in America, and Hicksite Quakers, among others.
Mainline churches share a liberal approach to social issues that often leads to collaboration in organizations such as the National Council of Churches. Because of their involvement with the ecumenical movement, mainline churches are sometimes (especially outside the United States) given the alternative label of ecumenical Protestantism. These churches played a leading role in the Social Gospel movement and were active in social causes such as civil rights and equality for women. As a group, the mainline churches have maintained religious doctrine that stresses social justice and personal salvation. Politically and theologically, mainline Protestants are more liberal than non-mainline Protestants.
Members of mainline denominations have played leadership roles in many aspects of life, including politics, business, science, the arts, and education. They founded most of the country's leading institutes of higher education. Marsden argues that in the 1950s:
- Mainline Protestant leaders were part of the liberal-moderate cultural mainstream, and their leading spokespersons were respected participants in the national conversation."
Some mainline Protestant denominations have the highest proportion of graduate and post-graduate degrees of any other denomination in the United States, such as the Episcopal Church (56%), the United Church of Christ (46%), and the Presbyterian Church (U.S.A.) (46%), as well as the most of the American upper class. Episcopalians and Presbyterians also tend to be considerably wealthier and better educated than most other religious groups, and they are disproportionately represented in the upper reaches of American business and law. From 1854 until at least 1964 they were heavily Republican. In recent decades, Republicans slightly outnumber Democrats.
Since the 1960s, however, mainline groups have shrunk as a percentage of the population as increasing numbers of Protestants have come to affiliate instead with fundamentalist, evangelical, or charismatic churches, or with no church at all. Mainline denominations peaked in membership in the 1950s and have declined steadily in the last half century. From 1965 to 1988, mainline church membership declined from 31 million to 25 million, then fell to 21 million in 2005. While in 1970 the mainline churches claimed most Protestants and more than 30 percent of the population as members, today they are a minority among Protestants; in 2009, only 15 percent of Americans were adherents.
The term mainline Protestant was coined during debates between modernists and fundamentalists in the 1920s. Several sources claim that the term is derived from the Philadelphia Main Line, a group of affluent suburbs of Philadelphia; most residents belonged to mainline denominations. Today, most mainline Protestants remain rooted in the Northeastern and Midwestern United States. Charles H. Lippy (2006) defines the term as follows: "the term "mainline Protestant" is used along with "mainstream Protestant" and "oldline Protestant" to categorize denominations that are affiliated with the National Council of Churches and have deep historic roots in and long-standing influence on American society."
Some have criticized the term mainline for its alleged ethnocentric and elitist assumptions, since it almost exclusively described white, non-fundamentalist Protestant Americans from its origin to the late twentieth century.
Mainline vs. mainstream
The term "mainstream Christian" in academic usage is not equivalent to "mainline Protestant" and is often used as an attempt to find non-loaded sociological vocabulary in distinguishing "orthodoxy" and "heresy." Hence in christological and doctrinal reference "mainstream Christianity" is often equivalent to "Trinitarianism." In Britain and Australia the term "mainline Protestant" is not used, and "mainstream" does not mean "liberal" Protestant.
- With almost 7.7 million members in the United States in 2009, the United Methodist Church is the largest mainline Protestant denomination
- The second largest mainline denomination is the Evangelical Lutheran Church in America (ELCA), with approximately 3.86 million members as of 2013
- Third in size is the Episcopal Church, with approximately 1.89 million members in the United States in 2012
- The fourth largest mainline denomination is the Presbyterian Church (U.S.A.), with approximately 1.76 million members in 2013
- American Baptist Churches USA are fifth in size, with approximately 1.3 million members (2008)
- The United Church of Christ has a little over 998,000 members in 2012
- The Christian Church (Disciples of Christ) has a little over 625,000 as of 2012.
- Religious Society of Friends (Quakers) 350,000 members
- Reformed Church in America 250,938 members (2009)
- International Council of Community Churches 69,276 members (2009)
- National Association of Congregational Christian Churches 65,392 members (2002)
- Moravian Church in America, Southern Province 21,513 members (1991)
- Moravian Church in America, Northern Province 20,983 members (2008)
- Universal Fellowship of Metropolitan Community Churches 15,666 members (2006)
- Latvian Evangelical Lutheran Church in America 12,000 members (2007)
- Estonian Evangelical Lutheran Church
- Congregational Christian Churches, (not part of any national CCC body)
- Moravian Church in America, Alaska Province
The Association of Religion Data Archives has difficulties collecting data on traditionally African American denominations. Those churches most likely to be identified as mainline include these Methodist groups:
- African Methodist Episcopal Church
- African Methodist Episcopal Zion Church
- Christian Methodist Episcopal Church
Some denominations with similar names and historical ties to mainline groups are not considered mainline. The Southern Baptist Convention, Lutheran Church – Missouri Synod, the Christian and Missionary Alliance (C&MA), the Churches of Christ, the Presbyterian Church in America (PCA), and the Anglican Church in North America (ACNA) are often considered too conservative for this category and thus grouped as evangelical.
Mainline churches hold a wide range of theologies—conservative, moderate and liberal. The inclusion of a denomination in the mainline Protestant category does not imply that every member of that denomination, nor even every member of their clergy, accepts some of the beliefs generally held in common by other mainline churches. They allow considerable theological latitude. Moreover, mainline denominations have within them Confessing Movements or charismatic renewal movements which are more conservative in tone.
About half of mainline Protestants describe themselves as liberal.
Nearly one-third of mainline Protestants call themselves conservative, and most local mainline congregations have a strong, active conservative element. Mainline denominations are historically Trinitarian and proclaim Jesus Christ as Lord and Son of God; they adhere to the historic creeds such as the Nicene Creed, the Apostles' Creed, and the Athanasian Creed.
In practice, mainline churches tend to be theologically moderate and influenced by higher criticism, an approach used by scholars to separate the Bible's earliest historical elements from perceived later additions and intentional distortions. Mainline denominations generally teach that the Bible is God's Word in function, but that it must be interpreted both through the lens of the cultures in which it was originally written, and examined using God-given reason. A 2008 survey conducted by the Pew Research Center found that only 22 percent of the 7,500 mainline Christians surveyed said the Bible is God's Word and is to be interpreted as literally true, word for word. Thirty-eight percent thought that the Bible is God's Word but is not to be taken literally, word for word. Twenty-eight percent said the Bible was not the Word of God but was of human origin.
The mainline denominations emphasize the biblical concept of justice, stressing the need for Christians to work for social justice, which usually involve politically liberal approaches to social and economic problems. Early in the 20th century, they actively supported the Social Gospel.
Mainline churches were basically pacifistic before 1940, but under the influence of people such as Reinhold Niebuhr they supported World War II and the Cold War. They have been far from uniform in their reaction to homosexual behavior, though generally more accepting than the Catholic Church or the more conservative Protestant churches.
The term "mainline" once implied a certain numerical majority or dominant presence in mainstream society, but that is no longer the case. Protestant churches as a whole have slowly declined in total membership since the 1960s. As the national population has grown they have shrunk from 63% of the population in 1970 to 54% by 2000. The mainline denominations slipped from 55% of all Protestants in 1973 to 46% in 1998. The number of mainline congregations in the U. S. declined from more than 80,000 churches in the 1950s to about 72,000 in 2008.
Various causes of mainline decline in population have been cited. Much analysis has taken place both from those within and outside mainline denominations. Key factors indicate that all types of churches can and do grow, regardless of hymnody or contemporary music, type of liturgy, average age of worshiper, or location  On average, however, churches in rural areas, churches with older congregants, and churches with less youth involved struggle most to add members and grow churches. For example, of all churches founded since 1993, 54% are experiencing growth, while that is true for only 28% of congregations founded prior to 1900. As demographics change, the churches founded by earlier generations often struggle to adapt to changing conditions, including the declines or shifts in the age and ethnicity of local populations. Says David Roozen, Director of Hartford Seminary's Hartford Institute for Religion Research, "Location, Location, Location used to be the kind way that researchers described the extent to which the growth or decline of American congregations was captive to the demographic changes going on in their immediate neighborhoods."  Age demographics cannot be overlooked as a real factor in congregational decline, with the birthrate for mainline Protestants well below what is needed to maintain membership numbers.
As congregational needs change, the institutions of mainline Protestant have struggled to adapt. The Barna Group, an Evangelical surveyor, has noted, Protestant pastors who serve mainline churches serve on average half as long as Protestant pastors in non-mainline churches. This may contribute to decline and is influenced in part by the United Methodist Church practice of Itinerancy, where clergy are intentionally moved from one church to another as often as yearly in an effort to support and encourage the United Methodist tradition of strong lay ministry. Mainline churches have also had difficulty attracting minorities, particularly Hispanics. Hispanics comprise 6 percent of the mainline population but 16 percent of the US population. The Barna Group, considers the failure of mainline Protestants to add substantial numbers of Hispanics to be portent for the future, given both the rapid increase of the Hispanic population as well as the outflow of Hispanics from Catholicism to Protestant churches in the past decade, most of whom are selecting evangelical or Pentecostal Protestant churches.
In general, however, decline can be a difficult thing to statistically quantify. Many older Protestant churches lived a vibrant lifetime and continue to evidence vital ministry and faith regardless of declining populations or birthrates. For example, giving and engagement with need and justice, both indicators of strong Christian faith, have increased despite the aging and loss of congregational members.
Contrast with conservative churches
While mainline churches have seen shrinking membership and worship attendance, both evangelical and fundamentalist Christian groups have been growing. About 40% of mainline Protestants in the 1990s were active in church affairs, compared to 46% of the conservatives.
Demographers Hout, Greeley, and Wilde have attributed the long-term decline in the mainstream membership and the concomitant growth in the conservative denominations to four basic causes: birth rates; switching to conservative denominations; departure from Protestantism to "no religion" (i.e. secularization); and conversions from non-Protestant sources. In their analysis, by far the main cause is birth rates—low for the mainline bodies, and high for the conservatives. The second most important factor is that fewer conservatives switch to mainline denominations than before. Despite speculation to the contrary, Hout, Greeley, and Wilde argue that switching from a mainline to a conservative denomination is not important in accounting for the trend, because it is fairly constant over the decades. Finally, conservative denominations have had a greater inflow of converts. Their analysis gives no support for the notion that theological or social conservatism or liberalism has much impact on long-term growth trends.
Evidence from the General Social Survey indicates that higher fertility and earlier childbearing among women from conservative denominations explains 76% of the observed trend: conservative denominations have grown their own. Mainline denomination members have the lowest birthrate among American Christian groups. Unless there is a surge of new members, rising death rates are predicted to diminish their ranks even further in the years ahead.
Some other findings of the Barna Group:
- From 1958 to 2008, mainline church membership dropped by more than one-quarter to roughly 20 million people—15 percent of all American adults.
- From 1998 to 2008, there was a 22 percent drop in the percentage of adults attending mainline congregations who have children under the age of 18 living in their home.
- In 2009, nearly 40 percent of mainline church attendees were single. This increase has been driven higher by a rise in the number of divorced and widowed adherents.
- From 1998 to 2008, volunteerism dropped 21 percent; adult Sunday school participation decreased 17 percent.
- The average age of a mainline pastor in 1998 was 48 and increased to 55 by 2009.
- Pastors on average remain with a congregation for four years compared to twice that length for non-mainline church leaders.
Recent statistics from the Pew Forum provide additional explanations for the decline.
- Evangelical church members are younger than those in mainline denominations. Fourteen percent of evangelical congregations are between 18 and 29 (compared to 2 percent), 36 percent between 30 and 49, 28 percent between 50 and 64, and 23 percent 65 or older.
Not paralleling the decline in membership is the household income of members of mainline denominations. Overall, it is higher than that of evangelicals:
- 25% Reported less than a $30,000 income per year.
- 21% Reported $30,000–$49,999 per year.
- 18% Reported $50,000–$74,999 per year.
- 15% Reported $75,000–$99,999 per year.
- 21% Reported an income of $100,000 per year or more, compared to only 13 percent of evangelicals.
Protestantism's hundreds of different denominations are loosely grouped according to three fairly distinct religious traditions—evangelical Protestant churches (26.3 percent of the overall adult population), mainline Protestant churches (18.1 percent) and historically black Protestant churches (6.9 percent).
The Association of Religion Data Archives ARDA counts 26,344,933 members of mainline churches versus 39,930,869 members of evangelical Protestant churches.
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