Ecclesiastical separatism is the withdrawal of people and churches from Christian denominations, usually to form new denominations.
In the 16th and 17th centuries, the separating puritans advocated departure from the Church of England. These people became known as dissenters.
Ecclesiastical separatism has also been associated with Christian fundamentalism, and such withdrawals have been mainly due to perceived theological liberalism. They have often been accompanied by a refusal to have any further association with the parent denomination or Christian fellowship with its members. George Marsden notes that Arno C. Gaebelein was one of the early fundamentalist leaders to advocate ecclesiastical separation in a conference address in 1914. Gaebelein had left the Methodist Episcopal Church in 1899. For Carl McIntire in the 1930s and 1940s, separation meant leaving liberal denominations (he formed the Bible Presbyterian Church) as well as organizations such as the National Council of Churches (he formed the rival American Council of Christian Churches). McIntire also separated from evangelical groups, such as the National Association of Evangelicals, which he believed had compromised with the liberalism of the National Council of Churches.
In fundamentalism, ecclesiastical separatism is closely connection to the doctrine of separation, in which Christians are urged to be personally separate from the world. This is often based on 2 Corinthians 6:17: "Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you." Dennis Costella bases his ideas of separation on God's holiness, and argues that this requires not just "withdrawal from counterfeit, apostate Christianity", but also "separation from disobedient brethren". The "refusal to associate with groups who endorse questionable doctrinal beliefs or moral practices" is known as "first-degree separation", while "second-degree separation" means "refraining from association or identification with groups or individuals who do not practice first-degree separation."