Congregationalism arrived in America twice within a decade, carried by two groups of English Protestants who agreed that a bishop had no proper place over a Christian congregation but disagreed about how completely to break with the Church of England over it. The Pilgrims who settled Plymouth in 1620 were Separatists, having already renounced the established church outright before crossing the Atlantic. The Puritans who founded the much larger Massachusetts Bay Colony beginning in 1630 were non-separating, hoping in principle to reform the Church of England from within even as they built, in practice, congregations that governed themselves entirely.
The distinction mattered less with each passing decade, and by 1648 the two colonies' ministers and lay delegates could gather at Cambridge, Massachusetts, and agree on a common statement of church order, the Cambridge Platform, which fixed the classic New England form: each congregation gathered by covenant, calling and ordaining its own minister, admitting its own members, and answerable to no higher ecclesiastical body, though neighboring churches might advise one another in council. Later adjustments followed as the colonies matured, the Halfway Covenant of 1662 easing membership requirements for the children of the unconverted, and the Saybrook Platform of 1708 introducing a modest, never fully embraced measure of consociational oversight in Connecticut. Through all of it, the basic Congregationalist conviction held: Christ governs his church through the gathered congregation itself, and no synod, bishop or presbytery may stand between them.