Was Luther a mystic? In places, he says positive things about mysticism. In other places, he "hurls the inkwell at the devil". His attitude toward mysticism, like his attitude toward many other things, is ambiguous. He is clearly attracted to the theology of the heart, that many mystics represent. He is not interested, however, in the extra revelation that he believes is part of mysticism. In this, he seems to have an epistemological, rather than a hermeneutical, understanding of mysticism.
Heinrich Bornkamm has given a number of ways in which he believes that Luther is distinct from the mysticism of Eckhart (contained in Ozment, The Age of Reform, p. 241). It is important to realize that these ways might also describe how the Friends are distinct from Eckhart:
So, Luther seems distinct from some of the concerns of at least Eckhart. But in other ways, he does seem to hold similar concerns to later Rhineland mystics. There is the concern for ethics as grounded spiritually.
At the same time as Luther published the Theologia Germanica, though, the Catholic church banned it. It is still on the Index, actually. But from some of the comments made so far, it should be easy to see the attraction of this little volume in Protestant circles. Luther is not the only one enthusiastic about this volume. It was a hit with the Anabaptists as well. Denck, Hetzer, Schwenckfeldt all thought highly of it. In the Reformed tradition, it did not have as strong an impression, but some (Sebastian Castellio, for example) found it a good antidote to the rigour of Reformed theology. Most in that tradition, though, saw no use in it. Calvin advised people to avoid the book, saying that it contained "deadly poison". And Theodore Beza (who followed Calvin in Geneva) also condemned it.
But the main work was done in Luthern circles. It should be said that Luther's enthusiasm did not mean that everyone accepted it wholeheartedly; the Reformed arguments held some force. And, as Lutheranism became more "orthodox", it became increasingly wary of this book. It was associated with such Lutheran heretics as Valentin Weigel. And, it seemed to have an ethical side which could have put it in the same camp as the book of James -- in other words, justification by some form of works.
But of course, that is not what Luther saw in it. The works were a result of justification, not a cause. And the stuff about the old and the new man sits well with the theology of justification. And, there is an emphasis on the inner life of the individual which makes good Lutheran sense.
But it should be noted that it is the Lutherans of the Pietist side, not of the orthodox side, that maintained the book's popularity. The book can be read as stressing personal piety and subjective spirituality. Gottfried Arnold, a leader of the Pietists, took Luther's enthusiasm over this book as an indication that the Pietist side was correct. Johann Arndt, another Pietist, cast the distinction as being between the "Christ in us" theology of the orthodox side, and the mystical theology of the Pietist side that was orthodox with respect to pure doctrine, but a proponent of the inner "heart" theology.
The point here is that theology is grounded in the here, not the hereafter. Righteousness happens not as an eternal reward, but as a present possibility. This means that there is some kind of mystical grounding of ethics that is possible. There are several themes that point to this:
1. Recognition of the good earth: There is a positive message about the earth and incarnate existence. The dualism is not between nature and spirit, but between self-will and God's will. So, God's presence is here on earth. There is positive importance given to material existence.
2. The Ethics of the Changed Soul: Instead of the soul being taken out of the world, the human soul is asked to make room for the living, present God. This can only be done by surrendering self-will. This change of disposition does not mean that we have simply added a moral code to our lives. Morality is a kind of necessary by-product of the change from the proud to the humble soul. But the surrendering of the self-will does not mean that the self is extinguished. Rather, it means that the self has God flowing through it. This leads to the path of surrender and cross-taking.
3. The Ethics of the Second Mile: The mystical life means that it is possible to have an ethics that involves suffering. The person can do the good thing, even if it is not personally comfortable. Luther talks this way, too. We live for and with others "under the cross". Christians then look like poor beggars, but they actually own everything.
4. The Place of Rule in Moral Life: The truly ethical is both rule-free and rule-bound. Note that this is against the Free Spirits. There are rules, because humans are still part of the outer world. But the other side is that the laws are internalized by the changed person. They have an inner meaning, and therefore are freely adopted.
5. Ethical Responsibility -- Carry and Be Carried: Morality is more than a rational decision. There has to be inner help from God. Therefore, a person will not be truly moral without this help.