Congregational churches are direct descendants of the Puritan movement in England. It originally adopted the theology of Puritanism and maintained church organization based on the supremacy of individual congregations. In 1648, the Cambridge Synod adopted the Westminster Confession, an extremely Calvinistic expression of dogma.
Congregationalism expanded due to the substantial immigration between 1640 and 1660. Harvard and Yale colleges were established to ensure the training of Congregationalist ministers. In 1708, the Connecticut Congregationalists adopted the Saybrook Platform which tended more towards Presbyterianism than their brethren in Massachusetts.
New England Congregationalism experienced further growth during the Great Revival (1734-1744). However, doctrinal issues split a number of churches. The ongoing disputes led to the emergence of Unitarianism in 1820.
Although the westward expansion of the nation included many Congregationalists from New England, the church did not have an organization for capitalizing on the new territories. As a result, Congregationalism remained largely a New England phenomenon.