From the little I’ve read so far, I’ve seen that Luther’s ecclesiology was what one might call functional. The Church was neither an invisibile platonic form as the Reformed taught that each peculiar church agrees or disagrees with to some extent, neither was it the eschatologically present and glorified victorious church of Rome and the Papacy, or the holy flock of the Anabaptists. For Luther, the Church was where the Word ruled sinner-saints who were neither morally superior to the world or a realized kingdom of Christ able to rule over the souls even of the dead (Purgatory & Indulgences). Where the Word is joined to water and bread in the sacraments, and where the Word of Law and Gospel are preached, there is the church. To make it an Ignatian axiom it might be: where the Word is, there is the Church.
It would’ve been curious to hear Luther’s thoughts on the Roman Catholic Church after the Second Vatican Council, after which all then heard the Scriptures in their own language. In the harshness of Luther’s words here, remember that this was before any reform in the Roman Church. He uses Noah and his family as a model for the Church in his commentary on Genesis.
“The Church is a daughter born from the Word, not the mother of the Word. Therefore, whoever loses the Word and looks to men instead, ceases to be the Church and lapses into utter blindness; nor will either great numbers or power avail. They who keep the word, as did Noah and his family, are the Church, though they be few in number, even but eight souls. The Papists at this time surpass us in numbers and rank; we not only are cursed, but suffer many things. But we must endure until the judgment, when God will reveal that we are his Church, and the Papists the church of Satan.
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Therefore though in their judgment they rob us of the title of the Church, still we retain the Word, and through the Word we have all ornaments of the true Church. For whoever has the Creator of all, must needs also possess the creatures themselves. In this sense Noah remained master of the world, though the waters prevailed, and the earth perished. Though he lost his property, yet, because he retained the Word by which everything was created, it may truly be said he retained everything.”