These are bewildering times for Christians seeking to live faithfully to Christ, under his authority. I think they are times which require us to rethink our approach to a number of things, not least how we understand Christian unity.
The approach to Christian unity which has characterised evangelicalism rests, I think, particularly on a formal principle: the authority of Scripture. We can unite with people who share our commitment to the authority of Holy Scripture. There is a lot of sense in this. Whilst we can have a conversation with all sorts of people, there is no likelihood of agreement where there is no common commitment to a way of knowing. Disagreements between people who are equally committed to Scripture at least have some hope of resolution, and an agreed way (in principle) of reaching that resolution: we read and study and debate Scripture together. Take away that formal agreement - either by taking away the commitment to Scripture, or by adding to it another authority - and material agreement becomes much more difficult, perhaps even impossible. At the very least, we are having a different sort of conversation if we're talking to someone who isn't happy to follow us into the Bible for answers, and who isn't pre-committed to accepting and submitting to those answers if they're satisfied that they really are biblical.
And so the authority of Scripture is a sensible rallying point. But it has never been the case that commitment to this formal principle alone is sufficient for Christian unity. There have always been heretics who claim to hold to biblical authority, and even make an impressive show of deference to the Bible. Leaving actual heresy aside, even amongst mutually acknowledged Christians there are limits to how much practical unity we can have purely on this formal basis. And so we qualify our basis for unity: we have unity with those who take Scripture as their authority (the formal principle) within certain bounds (and here we are introducing material beliefs). Normally for evangelicals that means there is a minimalist statement of faith which we look to as a standard; and so long as people subscribe our minimum standard, and remain committed to the formal principle, we allow latitude on a whole bunch of issues.
And here's where it gets tricky. Our minimum standards don't tend to address the hot-button issues of the day, like racism or human sexuality. The latter in particular is becoming a significant dividing line amongst professing Christians, and it isn't addressed in our evangelical standards. So what do we do? Typically we fall back here on the formal principle: you have to believe what the Bible says about sexuality. We turn it from a material issue (about theological anthropology, say) into a formal issue (about the authority of the Bible). But this raises two issues. Firstly, what do we do when people on the other side of the debate claim to be submitting to the authority of Scripture? We can debate them, in that case, on biblical grounds, and hope to persuade them of our reading of Scripture, but in the meantime is this an 'agree to disagree' situation? I don't see how it can be. Second, if this is in fact a fracture point, do we understand why it is so significant? Why must we divide over this disagreement about the interpretation of Scripture, but not over so many other things which we have (for the sake of unity) designated 'secondary issues', outside the scope of our doctrinal statements?
Here is a difficult thing: we don't want to divide over issues like baptism (who, when, how), or church government, or our understanding of eschatology, but we will divide over sexuality. Doesn't it sound like we're just cherry picking issues? Might it not seem as if this is driven basically by homophobia rather than doctrine? Why, after all, pick this issue as the line? It will not do to claim that sexual ethics is more important or central - more important than baptism, "which now saves you"? (Elevating anthropology and ethics above the church and soteriology is not a great way to go, I think). I am also not convinced it will do to claim that Scripture speaks more clearly on this issue - I think it is also perfectly clear on baptism!
It seems to me that the way forward is a renewed confessionalism, which will show that our formal principle is not merely formal, but carries material content. That is to say, we need to be able to show that Christian doctrine does not proceed in two stages - first sorting out the source of doctrine in Scripture and then moving on to what the Bible actually says. Rather, we need to show that our commitment to Scripture and its authority is part of a whole view of God's being and activity; that it already carries with it material content; that the nature of Scripture and its place within the dispensation of grace entails a particular way of reading. We need a thicker, more substantial doctrine of Scripture, along with a broader confession of Christian truth that goes beyond the bare minimum. Nobody wants to build higher fences unnecessarily, but I'm not sure we have any other option if we want to maintain Christian orthodoxy in our churches.