We are not talking about a platonic republic here, Melanchthon protested in the Apology. The Church, he insisted, has a concrete and real existence in this world.
But too often hasn't Lutheranism been reduced to a mind-set, a way of thinking, assent to a set of propositions? If you agree to X, Y, and Z, then, congratulations! You're a Lutheran.
Thus, Lutherans AGREE that private confession and absolution is a good thing. But don't ask them to do it! That would be "legalistic" (i.e., it would require moving out of the realm of ideas).
Thus, Lutherans AGREE that Baptism means death to the old Adam. But don't ask them fast! That would be "legalistic" (i.e., it would require moving out of the realm of ideas).
Thus, Lutherans AGREE that the Symbols are a "true exposition of the Word of God" but don't refer to Mary with any of the titles that they do ("Blessed, Ever-Virgin, Most-holy, Pure and Holy", etc), or you're imposing personal beliefs on people (and you'd be moving out of the realm of ideas).
Thus, Lutherans AGREE that the Saints are to be honored, but do it without mentioning them, please! We don't want to be mistaken for Rome (in other words, keep it in the realm of ideas, not practice).
Thus, Lutherans AGREE that there is a third use of the Law, but God forbid that a pastor expect a person to actually follow one of the commandments! The Law is there to accuse and that's about it, remember? That keeps it nicely in the realm of ideas. Safely away from my life!
I may be being too harsh (it certainly would not be the first time), but it seems to me watching and listening and thinking about theological discourse of late, that any time the discussion moves away from the realm of mere ideas, the typical Lutheran response among many so-called "Confessionalists" is to squirm with discomfort and cry foul.
Can Lutheranism survive merely as an idea? Can the idea of Lutheranism be disconnected from the concrete forms in which she has lived her life? I am thinking of how it was reported to me that a professor at one of our seminaries can argue that both Fr. High Church and Pastor Billy-Bob Church Growther are both fine and upstanding Lutheran pastors. Why? Because they hold to the same ideas, they hold opinions that mark them as Lutheran. Lutheranism as an idea is not Lutheranism at all in my book; unlike Sasse, I agree with Elert, there is a morphe to Lutheranism, a shape that results from the impact of the Gospel. Take away that concrete shape and all you have left is the Gospel as an idea.
I am reminded of the words of St. Basil the Great: "If we attacked unwritten customs, claiming them to be of little importance (read: adiaphora), we would fatally mutilate the Gospel, no matter what our intentions - or rather, we would reduce the Gospel teachings to bare words (read: ideas)." par. 66 *On the Holy Spirit*
I am afraid the great Father nailed that one. And it is why Article XV of the AC is a DOCTRINAL article.