Do you have an original source for that quote? I haven’t yet found a manuscript titled “Commentary on St. John” by Luther though that is the general description given for it.
No replies i see, but this uncritical RC quoting of quotes by Luther abounds, and even when they have a actual source, they are usually taken out of context. Thus i provided that of some quotes which a certain poster are pasted thrice today.
And therein i provide the best source for researching such quotes, and in which can be seen one of at least two posts by Swan addressing this statement, and sourced to Luther’s Sermons on John 16 [LW 24 (Luther's Works in English)]
Hope this helps.
And actually, Gamecock had just posted that article: http://www.freerepublic.com/focus/religion/3131080/posts?page=10