New England Congregationalism, though already disturbed by the theological controversy which later produced the Unitarian defection, was in the main soundly Calvinistic. It differed from Presbyterianism only in its tradition of the independence of the local church, and even this was qualified by the growth of what was called "associationism" by those who viewed it with alarm. So, when an interest in home missions began to appear, about 1800, the Plan of Union was formed under which Congregationalists and Presbyterians cooperated until 1837 in carrying the gospel to the new settlements, first in western New York and then in the regions beyond. The Presbyterians ultimately got most of the churches organized in the Middle West by Congregational missionaries operating under this plan.
Presbyterians came from England, Scotland, and North Ireland. They never had a colony of their own, though they missed having Massachusetts Bay only because the Presbyterian Puritans who founded it became Congregational. Puritans who came to other colonies generally were and remained Presbyterians. They found a footing in New York, New Jersey, Maryland, Virginia, the Carolinas, and Georgia and were among the first settlers of Kentucky. Pennsylvania became the scene of some of their most vigorous activities, both in and around Philadelphia and in the central and western part, where they were the most numerous and influential group. William Tennent's "Log College" at Neshaminy (1720) initiated theological education in America, at least outside of Harvard's effort to provide a learned clergy for New England. It trained evangelists as well as scholars, and led to the founding of Princeton. The great Scotch-Irish immigration, about the middle of the eighteenth century, brought both regular Presbyterians, in communion with the Church of Scotland, and Seceder Presbyterians, representing the Great Secession of 1733. Large numbers of both came to the western parts of Pennsylvania and Virginia, where these Presbyterian Ulstermen "formed an American Ulster larger and richer than that they had abandoned," as one of them wrote, with some exaggeration of the degree of their occupancy though not of the size and resources of the area. Thomas Campbell was following a stream of Scotch-Irish Seceder Presbyterians when he migrated from the vicinity of Belfast, Ireland, to the southwestern corner of Pennsylvania.