|Preach to the choir|
|Crux of the matter|
|Speak of the devil|
|An act of faith|
Ecumenism (aka Ecumenicalism or the Ecumenical movement) refers to the beliefs and practices of certain religionists who have the crazy idea that disparate religions — i.e., Baptists, Mormons, and Catholics or Christians, Muslims, and Jews — should set aside their
ungodly hatred for each other differences and work together for world peace and social justice. It also refers to movements specifically within Christianity to unite different denominations under an umbrella body, or even to merge them into a single denomination. The latter has been notably successful in Canada, with its largest Protestant denomination being the United Church of Canada, a merger of the Methodist, Congregationalist, Presbyterian, Local Union, and Evangelical United Brethren churches.
Ecumenism had its roots in the Christian missionary field, where missionaries of different sects found themselves together in the field and had to reconcile some of their differences in order to get the job done. The first notable step towards ecumenism came in 1910 with the gathering of differing Protestants at the International Missionary Conference. Today, there exist churches that unite splintered sects, such as the United Church of Christ and Evangelical Lutheran Church of America. The National Council of Churches of Christ in the United States, and the World Council of Churches, are two major Christian ecumenical organizations.
Fundamentalist Christians view ecumenism as a movement of liberal denominations to compromise what they view as essential Christian doctrine such as creationism, the virgin birth, a literal hell, and the inerrancy of the Bible, all in the name of setting aside their differences. They may view it as a movement whose end result is muting the message of sin and personal salvation, and replacing it with a "social gospel" of secular political activism around mostly liberal issues such as social justice, ending poverty, opposition to war and the death penalty, etc.
Those fundamentalists of an anti-Catholic bent often adhere to a conspiracy theory that the ecumenical movement is a plot by the Pope to merge all the Protestant churches back into the Catholic Church.
Fundamentalists obsessed with the end times likewise often adhere to a belief that ecumenism will be the vehicle through which the Antichrist establishes a world "superchurch", which denies all the essentials of the Christian faith while fooling most people into thinking it is a Christian church. (What the fundies don't realize is that they're really describing themselves.)