Perhaps not from its inception, but certainly in the post-WWII era, the global Christian ecumenical movement, as represented by groups like the World Council of Churches, the Lutheran World Federation, and the World Alliance of Reformed Churches, has been increasingly dominated by Marxist economics, liberation theology, and transformationalist ethics.
Much of this was mediated through the influence and work of Reinhold Niebuhr. Niebuhr in part observed the reality that since there was no single government above nation-states which could restrict and regulate their activity, the realm of global realpolitik is doomed to be characterized by immorality and warmongering.
If “all social co-operation on a larger scale than the most intimate social group requires a measure of coercion,” and “If, as Bertrand Russell prophesies, some form of oligarchy, whether capitalistic or communistic, be inevitable in a technological age, because of the inability of the general public to maintain social control over the experts who are in charge of the intricate processes of economics and politics, the communistic oligarch would seem to be preferable in the long run to the capitalistic one. His power would be purely political, and no special economic interests would tempt him to pursue economic policies at variance with the national interest .”
No doubt in its utopianism, idealism, and therefore almost exclusive blame for the ills of the world upon global capitalism the ecumenical movement has gone far beyond what Niebuhr himself had or ever would say (for, after all, unlike WARC, Niebuhr wrote, “Neither is it true that modern wars are caused solely by the modern capitalistic system with its disproportion of economic power and privilege.” He was a bit more nuanced).
Update: See also, “Reinhold Niebuhr is Unseen Force in 2008 Elections” and Reinhold Niebuhr Today.