After tracing the history of Protestant church union movement in China and the evolution of the National Christian Council of China (NCC), this study focuses on the troubled relationship between the China Inland Mission (CIM) and the NCC, and unveils the causes behind CIM's withdrawal from the NCC in 1926. CIM's act shows that the church union movement became one of the major battlegrounds between the fundamentalist and modernist missionaries, and the so-called "Protestant Mission Consensus" formed in the nineteenth century could no longer be sustained. Furthermore, most of the fundamentalists embraced the goal of Christian unity, but insisted on a conservative doctrinal basis for such a unity. Therefore, the fundamentalist-modernist conflict over the NCC was not simply one between "separatists" and "unionists", but one between the two different visions of Christian unity.