Brad does an excellent job of explaining how using historical resources can help us navigate between, on the one extreme, a call for hyper-precision in matters of dogmatic theology, a call which leads to constant separations and, at the other extreme, an indifference to Christianity's core beliefs in the vain hope that all our problems will disappear if we sit around the campfire singing kumbaya.
Distinguishing the doctrines that are essential from those that are peripheral is not itself a peripheral matter. Neither, however, is it one that is easily resolved. Developing a doctrine of "essentials" is not a DIY project. Brad explains that one important tool in the winnowing process is weighing the varying judgments in the Christian tradition with respect to what counts as an essential.
By analogy to what I explain to my Contracts students, identifying what is a Christian essential is something like finding the ratio decidendi among the several oral judgments of an English appellate court. Just as the lawyer confronted with the transcripts of oral decisions rendered without much regard to each other must look hard to find the common legal reasoning among the judgments that agree in result, so too must the contemporary Christian community compare the theological conclusions of its forebears to find what has been held in common across the spectrum of time and place.
Other matters of theological conclusion--what I've labeled as peripheral--are not unimportant. The process of doing theology is not done. It is, however, important to acknowledge that, just as there is more theologizing to do, that there is an "essential" center that should be acknowledged by anyone claiming to be part of the tradition called Christian.