First, we need to recognize that some doctrines are more important than other doctrines. All Christians recognize this to some extent. For example, the doctrine of the return of Jesus Christ, the second coming, is much more important than the question of whether Christ will return before or after the tribulation. If you deny the second coming of Christ, it calls into question whether you are a Christian. But Christians have always disagreed about the exact timing of Christ’s return. So which doctrines are more important and which are less important?
One way to think about this issue is to distinguish between three levels of doctrines. First level doctrines include those a person has to believe in order to be a Christian. These include things like the inspiration of Scripture, the divinity of Jesus Christ, the humanity of Christ, the sacrificial death of Christ for our sins, and his bodily resurrection. Now I am not saying that every Christian understands these doctrines fully. But if a person rejects these doctrines, can they really be a Christian in any historic sense?
Second level doctrines include those which are important because they promote the health of the church.
I recall the arguments over homosexuality in TEc went through a stage of questioning whether or not they belonged in a one level of doctrinal dispute or another. Thinking back, it would seem that the second or third level arguments won.
I still think that in order for the question to come up at all, there must be a first level problem which is probably the authority of scripture or a disagreement over the divine inspiration of scripture.
The non-renewed haven’t the word “adiaphora” at their disposal? It was certainly useful to those in the PEcUSA who wanted to label all their innovations as “second or third level” disagreements. And you see where it got the Epicopal entity – a byword for formerly Christian, now political action, groupies.