Acknowledgment that a mark of the church was that it was "one" was set forth in the Nicene-Constantinopolitan Creed finalized at the second ecumenical council (Constantinople; A.D. 381). For an extended discussion of this issue, see "The Four Positives of the True Church."
In order to tell the Church, it must be both known and visible. There cannot be a multiplicity of different churches or a strictly mystical entity. And the doctrine that is taught must be a single, coherent whole as well. Otherwise a single coherent answer could not be delivered and heard.
