Congregationalism is not limited only to organization of Christian congregations; the principles of congregationalism have been inherited by the Unitarian Universalist Association. Jewish synagogues in the U.S. operate under congregational government as well, with no hierarchies.
Before embarking on the theological foundations of congregationalism, it is also noteworthy that the earmarks of Congregationalism can be traced back to the Pilgrim societies of the United States in the early 1600s. Congregationalism expressed the viewpoint that (1) every local church is a full realization in miniature of the entire Church of Jesus Christ; and (2) the Church, while on earth, besides the local church, can only be invisible and ideal. While other theories may insist on the truth of the former, the latter precept of congregationalism gives the entire theory a unique character among plans of church government. There is no other reference than the local congregation for the "visible church" in Congregationalism. And yet, the connection of all Christians is also asserted, albeit in a way that defenders of this view usually decline, often intentionally, to elaborate more clearly or consistently. This first, foundational principle by which congregationalism is guided results in confining it to operate with the consent of each gathering of believers.
Although "congregational rule" may seem to suggest that pure democracy reigns in congregational churches, this is seldom the case. It is granted, with few exceptions (namely in some Anabaptist churches), that God has given the government of the Church into the hands of an ordained ministry. What makes congregationalism unique is its system of checks and balances, which constrains the authority of the minister, the lay officers, and the members.
Most importantly, the boundaries of the powers of the ministers and church officers are set by clear and constant reminders of the freedoms guaranteed by the Gospel to the laity, collectively and individually. With that freedom comes the responsibility upon each member to govern himself or herself under Christ. This requires lay people to exercise great charity and patience in debating issues with one another and to seek the glory and service of God as the foremost consideration in all of their decisions.
The authority of all of the people, including the officers, is limited in the local congregation by a definition of union, or a covenant, by which the terms of their cooperation together are spelled out and agreed to. This might be something as minimal as a charter specifying a handful of doctrines and behavioral expectations, or even a statement only guaranteeing specific freedoms. Or, it may be a constitution describing a comprehensive doctrinal system and specifying terms under which the local church is connected to other local churches, to which participating congregations give their assent. In congregationalism, rather uniquely, the church is understood to be a truly voluntary association.
Finally, the congregational theory strictly forbids ministers from ruling their local churches by themselves. Not only does the minister serve by the approval of the congregation, but committees further constrain the pastor from exercising power without consent by either the particular committee, or the entire congregation. It is a contradiction of the congregational principle if a minister makes decisions concerning the congregation without the vote of these other officers.
The other officers may be called "deacons", "elders" or "session" (borrowing Presbyterian terminology), or even "vestry" (borrowing the Anglican term) — it is not their label that is important to the theory, but rather their lay status and their equal vote, together with the pastor, in deciding the issues of the church. While other forms of church government are more likely to define "tyranny" as "the imposition of unjust rule", a congregationally-governed church would more likely define tyranny as "transgression of liberty" or equivalently, "rule by one man". To a congregationalist, no abuse of authority is worse than the concentration of all decisive power in the hands of one ruling body, or one person.
Following this sentiment, congregationalism has evolved over time to include even more participation of the congregation, more kinds of lay committees to whom various tasks are apportioned, and more decisions subject to the vote of the entire membership.
Since the formation of the United Church of Christ in 1957, most of the former CC churches largely follow a more centralized system coordinated by their associations and conferences to help them procure pastors or send candidates into the ministry from their churches, one largely designed to ensure a professional, credentialed pool of clergy. Meanwhile, the National Association of Congregational Christian Churches and the Conservative Congregational Christian Conference, two bodies formed in reaction to the dominant liberalism (in the first instance, political; the second, theological) of the majority of Congregationalists, have retained something closer to the older, more autonomous practices where associations do not supervise clergy.
Some other mainline groups governed by congregational polity, such as the American Baptist Churches USA, the Alliance of Baptists, and the Christian Church (Disciples of Christ), employ a system of clergy placement and supervision similar to that of the UCC.
An older, competing, but somewhat related theory, is sometimes called nationalism (in the Reformed churches tradition), or autocephaly (in the Eastern Orthodox Church tradition). Between these latter two there are further differences. In both nationalism and autocephaly, one unifying doctrine is given local expression, according to differences in language and customs. Autocephaly is strictly episcopal, and assures the self-government of distinct patriarchates within a structure of common doctrine, comparable practices, with some degree of mutual accountability through which they remain in communion with one another. In nationalism (in recent times, more accurately called "culturalism"), there is no institutional accountability to churches with separate general assemblies, although churches with separate histories typically form voluntary confederations with one another. Congregationalism, in contrast, guarantees a completely independent government for all of the uniting parties, down to the level of every local congregation.
The congregationalist principles of complete autonomy and strictly voluntary union produces a practically indescribable diversity of beliefs within the congregational unions. The UCC is the result of a union constructed according to congregationalist theory between the Evangelical and Reformed Church and the Congregational Christian Churches. These uniting congregations were themselves the result of several previous unions. The General Council of Congregational Christian Churches was formed from a merger between the National Council of Congregational Churches and the General Convention of the Christian Church, also known as Christian Churches or Christian Connection (not to be confused with, although partially related to, the Disciples of Christ). The Evangelical and Reformed Church was the result of a partial union of the Reformed Church in the United States and the Evangelical Synod of North America (a union of Lutherans and Reformed). The UCC is by far the most diverse of the Reformed churches in the U.S. In the United Kingdom, the United Reformed Church was formed in 1972 by the merger of the Presbyterian and the Congregational churches, on presbyterian principles of union but within a continuing congregational regard for local diversity.
In theory, the UCC is tolerant of all types of theology. Paragraph 18 of the UCC constitution is a provision that was specifically requested by the Congregational Christian Churches, and it declares,
These churches have developed ideas about independence of congregational authority that are quite different from the UCC and its New England-heritage predecessors, usually deriving from intense convictions about supposed patterns of government among the early churches described in the New Testament. Both traditions were also shaped culturally by the agrarian traditions in the rural South from the late 19th through the middle 20th centuries, the poverty of which largely did not permit much of the population to acquire significant formal education. These conditions enabled preachers to disseminate highly sectarian viewpoints such as the Landmarkist Baptist movement and doctrinare Restorationism without significant opposition on the part of potential converts.
Independent Baptist churches and Churches of Christ typically do not condone the theories of unity and "merger" outlined above, as such consolidation constitutes a threat to the sovereignty of individual congregations. Interdenominational unity is generally eschewed; calls for tolerance are almost invariably viewed as attempts to weaken their distinctive doctrines and separatist ecclesiologies. Church government beyond the level of the congregation does not exist. Even in small towns or in rural areas, most independent Baptist and Churches of Christ preachers do not meet on a regular basis. Preachers are not formally ordained in the Churches of Christ, because of the belief that performance of such a rite would constitute a transcongregational authority. Practices likely vary on this point among independent Baptists, according to local customs.
The CoC base these convictions upon their general consensus that the New Testament practice of epistle-writing had some practical, doctrinal, or interpretational authority because the letters were written by apostles and/or those directly inspired by God, but do not retain similar authority in modern times. The practice of writing rather than meeting is what gave rise to a colloquial maxim that "Churches of Christ don't have Bishops; they have editors instead." These editors (usually elders) publish such magazines as the Gospel Advocate and the Herald of Truth. Other than these editors and the occasional lectureship (in which preachers from many churches come together to speak publicly on pressing issues), the only ways in which Churches of Christ generally coordinate is in disaster relief.
Such rigorous independence even extends to some CoC-founded colleges, such as Florida College, which does not accept donations from churches for fear of undue influence and because the college's leaders hold to a policy that strict adherence to certain Biblical passages does not permit churches to donate money to education. However, most mainstream universities and colleges affiliated with the Church of Christ, such as Pepperdine, Harding University, and Lipscomb University, do accept money from churches; the schools, in turn, generally reflect the peculiar doctrinal platforms of those contributing churches. The Churches of Christ have steadfastly refused to organize along national or regional lines.
As for Baptists, a variety of parachurch agencies and evangelical educational institutions may be supported generously or not at all, depending entirely upon the local congregation's customs and predilections. Usually doctrinal conformity is held as a first consideration when a church makes a decision to grant or decline financial contributions to such agencies, which are legally external and separate from the congregations they serve. These practices also find currency among non-denominational fundamentalist or charismatic fellowships, many of which derive from Baptist origins, culturally if not theologically.
Most Southern Baptist congregations and African-American Baptist churches, by contrast, generally relate more closely to external groups such as mission agencies and educational institutions than do those of independent persuastion. However, they adhere to a very similar ecclesiology, refusing to permit outside control or oversight of local affairs.