I do not believe it all that complicated. There is a word for what the major streams of Christian thought hold in common, whether Catholic or Eastern or Reformed or Armenian, and subscribed to in the central historic creeds, and that word is usually understood to be orthodox.
Orthodox should not be taken as meaning correct. There is a cacophony of sects which maintain the creeds to be wrong or not in keeping with the original faith. The antonym of orthodox, heretical, has been elastically bounded. Nor is there a language police which patrol unorthodox usages of orthodox, so anybody can use the word how they will. But with all that in mind, it is still a useful enough word for conveying the most essential elements of Christianity.