The law of proclamation is indeed the law of doctrine. But to claim, while preaching, that the preached Word is indeed content-specific, and that one can either preach rightly or wrongly, and that in fact certain things are central to the faith and not others- these are things that need to be done in the face of ecumenical white-washing. Christ is our righteousness, in this we have unity. We don't have unity aside from or apart from this, and we surely have no unity if we need to vacate the faith of particular propositions. This would be an injustice to the clear articulation of the conclusion of the Athanasian creed. We are to believe thus-and-so, and not otherwise, and it is the job of preachers to proclaim the thus and so, in the Spirit of Christ.
I agree wholeheartedly with Greg's call to preach Christ as our righteousness rather than self-righteously claim purity of doctrine. Yet it is this purity of doctrine, the rightness of our believing that, in a sense, God reckons this faith as righteous, that is, the source and norm of our faith itself, and therefore for the sake of those hearing, we are to preach purity of doctrine and continue saying, "This and no other gospel- as if there were other gospels".
I remember a number of years ago, my internship supervisor came into work of a Wednesday morning, and said, "I just re-read Melanchthon's apology to Article 4 of the Augsburg Confession. How clearly and reasonably they wrote and thought." What we need as much as right preaching on this article is right thinking, and we can learn much from Melanchthon of right and clear thinking. I thus commit to re-reading, be it ever so long.