I am looking for some primary source texts that deal with Luther’s writings on theosis and divination. If anyone could help me out I would appreciate it. Thanks!
You may find what you want by consulting Project Wittenberg.
The “Freedom of a Christian” is one of the texts advocates of this “Finnish interpretation” cite–it uses language of nuptial union. Of course there are many others–your best bet might be to start with the secondary source Union with Christ: The New Finnish Interpretation of Luther edited by Carl Braaten and Robert Jenson, and see what primary sources (other than “Freedom”) are cited there. As this review points out, the passages most supportive of “theosis” tend to be in the earlier texts–the teaching of the Galatians commentary, for instance, is probably less conducive to this interpretation.