We are asked this question often enough that it seems useful to have our answer in writing!
Since the 17th century, Presbyterianism has been a branch of the Protestant Reformation which upheld Biblical orthodoxy, promoted missions and emphasized God's sovereign initiative in authoring salvation by grace through faith in Christ. Presbyterians were usually known for well-ordered church government, concern for doctrine, and reverent dignity in public worship.
Tragically in the 20th century, Presbyterianism was divided into several camps, over the basic issue of whether the written Word of God really is our infallible authority. During the so-called Fundamentalist vs. Modernist controversy of the 1920s, many mainline Presbyterian churches became captive to leaders who bluntly rejected orthodox Christianity. Miracles were denied; Scripture was blatantly undercut; God's wrath for sin was swept away; the necessity of personal salvation was de-emphasized; "ministry" became a matter of changing an external social order rather than preaching good news of the divine Christ who transforms souls eternally.
The PCUSA is still the largest Presbyterian body. As of 2015, this denomination claims two million members, yet it shrinks annually. Young adults have departed in droves, either to no church, or to evangelical churches. Beginning in the 1960s and '70s, many ministers and lay persons felt compelled by conscience to depart from the PCUSA, no longer finding it a home compatible to evangelical faith. This happened for me in 1980, when in the PCUSA, a man was ordained a minister by his presbytery and later was approved by the denomination's highest court, even though he explicitly denied the deity of Christ. It is not shocking that one man could express such heresy, but a majority of fellow pastors and elders across the denomination officially tolerated it! That was the final straw for me. To leave the church that ordained me was a decision I reached with great sadness.
In 1973, the PCA began in the southern states as a conservative breakaway movement. Primary issues related to the mainline church continued: compromises with the Word of God and aggressive promotion of liberal secularism. In more than 40 years of its existence, the PCA has grown from 40,000 adherents in one region, to about 380,000 in nearly all parts of the nation and Canada. We have been blessed by God with rapid growth. Our own Westminster congregation is bursting with about 80-90 new members each year, all hungry for forthright Bible teaching. In 1982 a happy union came about with the Reformed Presbyterian Church, Evangelical Synod, which broadened our base of churches nationally. The PCA continues to actively plant new churches on an average of about three a month across the USA and Canada. More than 40% of all PCA congregations are less than 25 years old, due to church planting.
The PCA, although a relatively small denomination, puts into the foreign field the world's largest Presbyterian missionary force, carrying good news of redemption in Christ to many nations. Missionary outreach in obedience to the Great Commission is a high priority. We sponsor a rapidly growing campus ministry at many colleges and universities called Reformed University Ministries, and have numerous military chaplains in all branches of the U. S. Armed Services.
PCA ministers and officers take a solemn vow to affirm the inerrancy of the Bible as their supreme authority and must endorse the entire Westminster Confession of Faith as a secondary creed summarizing the doctrines of Scripture. We believe laxity toward God's written Word opens the Pandora's box of errors humanistic theology has unleashed. People are eternally lost and need a divine Savior. Our pulpits seek to expound the Bible as transforming truth; we do not major in politics, faddish pop-psychology, or sociology.
Ministers, ruling elders, and deacons in the PCA are men only, in obedience to the New Testament standard for those who rule the church and teach doctrine, though women have a wide range of use for their gifts in our churches. The PCUSA, by contrast, has been drastically swayed by the overall feminization of American culture. The PCUSA endlessly debated homosexual ordination, which is not debatable in the PCA, since Scripture clearly condemns active homosexual practice as sin to be repented of by God's enabling grace. In 2011, the PCUSA approved ordaining persons in an active gay lifestyle as ruling and teaching elders. In light of Scripture, this is an abomination. While the PCUSA speaks predominantly for "pro-choice" on abortion, the PCA is unilaterally "pro-life," believing life begins at conception. The PCUSA is a major supporter of the World Council and National Council of Churches. The PCA has no participation in either organization.
In the PCA, all church property belongs irrevocably to the local church, but in the PCUSA if a congregation votes to leave, the real assets of local churches become property of the denomination. (Many PCUSA lay members do not even realize this!) PCA congregations do cooperate in relationships with other sister churches through presbyteries and general assembly. However, this is a voluntary relationship from which any congregation can withdraw by a 2/3 congregational vote, without penalty. We consciously avoid a top-heavy denominational system that gives manipulative power to a national "headquarters" staff, who generally tend to become detached from concerns of people in local congregations.
We in the PCA are interested in preaching and living out in a positive manner the gospel of our great God and Savior. Throwing brickbats at the PCUSA is not our favorite preoccupation. We don't even bring the subject up very often, as I have here. Self-righteousness is a deadly sin to guard against. The PCA is not a perfect church and is filled with sinners - of whom I am chief! We all have daily need of redeeming grace in Jesus Christ.
In summary, the main similarity between the PCUSA and PCA is a historic system of church government based on rule by elders and common traditions tracing to the Reformation. However, in biblical doctrine, evangelism, missions and social policy, the PCA is visibly more like other evangelical churches which take the Bible seriously and quite unlike "mainline" Protestant ecumenical churches, including most (but not all) congregations of the PCUSA. These distinctions are not superficial, but are rooted deep in the vitals of Christian faith.
You must decide who is right about these issues. In the last great Day, Christ the Bridegroom of the church will decide. To Him alone be the glory.
Dr. Michael A. Rogers,
Westminster Presbyterian Church