I think that Luther was struggling to distinguish between Christianity and Christendom, at least in the years leading up to the Leipzig Disputation (1519). If you try to distinguish them, you have to define them. I think part of Luther’s struggle was to distinguish by defining; or define by distinguishing. It was at Leipzig that he realized he really agreed with Jan Hus on things for which the Bohemian had been condemned. This set Luther at odds both with a Church Council and the Pope. It established relations between him and the Bohemian church. And what did the Pope stand for? No longer simply for true Christianity. Communion with the Pope was no longer the same as fidelity to Christ. What was Charles V dedicated to? The unified Holy Roman Empire. They made common cause against Luther because their cause was a common one: Christendom, which they also confused with Christianity. And Luther’s reformation was made politically viable because the Christian nobility of the German nation (what German Nation?) found the distinction useful. Of course they wanted to preserve a Christendom. But this is not the same as preserving Christendom.
Which is not to say that Christendom is in itself a departure from Christianity. Not entirely. It was the temporal manifestation of a society which had no other spiritual bond than the Christian religion, which formed its structures and institutions on the basis of its Christian identity, and which was in many ways glorious. Christendom is Europe, and Europe both before and after the Reformation was a great thing. But after the Reformation Europe is an unraveling Christendom (Here some careful consultation of Dawson will come in handy: The Formation of Christendom, The Dividing of Christendom).
John Eck affirmed that the unity of Christendom depended on the Pope’s exclusive headship. Luther denied it. He knew there were Christian societies—Byzantium—which had flourished without the Pope or a unified religious hierarchy. Nor did he believe Scripture warranted the Pope’s exclusive hegemony. That is to say: Christianity admitted to churches, not just one Church.