Second, I think we fail to consider that the battles over orthodoxy in the first few centuries occurred between people who had much more agreement among themselves on core Christian beliefs than they did with the surrounding pagan culture. For instance, the sixth ecumenical council (Constantinople 3, 680–681), which ruled on the issue of Monothelitism, was parsing very fine distinctions about the nature of Christ’s two natures. All the participants could plausibly say, “Hey look, we’re all Nicene and Chalcedonian Christians here.” Someone from the outside would certainly have seen their disputes as distinctions with barely a difference.
You had a dispute between Christians who affirmed “the supernatural tenets of the gospel,” grace, the resurrection of Christ, the Son’s consubstantiality with the Father, and so forth, yet this very fine distinction about Christ’s two wills was deemed a marker of orthodoxy because if it wasn’t affirmed, it functionally and materially undermined the rest of orthodox Christian doctrine. At this ecumenical council, some Nicaea- and Chalcedon-affirming Christians believed other formally Nicaea- and Chalcedon-affirming Christians were nonetheless promoting heresy. No one sets out to create heresy, but some sincere attempts to explain our faith end there. I think we need to consider the reality of theological controversy in history when we think about Smith’s questions and our unwillingness to use strong language about the issue.
As I said, though, I don’t mind using a different term, so long as we all agree that orthodox means only “signs off on the right propositions on some foundational issues settled by church creeds and definitions.” But what needs to be made absolutely clear at that point is that orthodoxy would then be an extremely limited concept for determining ecclesial boundaries and distinguishing normative Christian belief and practice. Orthodoxy would be necessary but nowhere near to sufficient for flagging the totality of beliefs within the acceptable spectrum of normative Christianity.
Let me put it this way: given this limited view of the term orthodoxy, it would be a coherent statement to say, “Joe is an orthodox Christian who believes adultery can be Christian behavior.” Or, “Joe is an orthodox Christian who believes bearing false witness can be Christian behavior.” Or, “Joe is an orthodox Christian who believes coveting can be Christian behavior.” None of those statements is incoherent unless “orthodox” just means “formally aligns on key Nicene and Chalcedonian propositions.” Yet it’s obvious in each case, somewhere Joe is severely out of line with the gospel. My point is that whatever extra term we might use to discuss the acceptability of SSM, that term would need to have some real, normative force.
Otherwise, while we may say this isn’t an issue of indifference, the more we repeat sentences like “Well, this is an argument between orthodox believers,” the more we begin to hear, “Well, this is a discussion between believers who are all basically in line with the gospel.” Smith’s thin definition of orthodoxy would still carry the thicker connotation it has typically had with all its boundary-defining force. Use of the term would become a theological bait-and-switch, or a reverse motte-and-bailey, in which we start with a modest claim and subsequently substitute it for a stronger one.