History of Christianity
Ecumenical movements within Protestantism have focused on determining a list of doctrines and practices essential to being Christian and thus extending to all groups which fulfill these basic criteria a (more or less) equal status, with perhaps one's own group still retaining a "first among equal" standing.  This process involved a redefinition of the idea of "the Church" from traditional theology.  This ecclesiology, known as denominationalism, contends that each group (which fulfills the essential criteria of "being Christian") is a sub-group of a greater "Christian Church", itself a purely abstract concept with no direct representation, i.e. no group, or "denomination", claims to be "the Church".  Obviously, this ecclesiology it at variance with other groups that indeed consider themselves to be "the Church".  But moreover, because the "essential criteria" generally consist of belief in the holy Trinity, it has resulted in strife between these Protestant ecumenical movements and non-Trinitarian groups such as Latter-day Saints (Mormons) and Jehovah's Witnesses, which are not often not regarded as Christian by these ecumenical groups.