Luther does not give all books of the the Bible equal weight. He evaluates them in their gospel content. John, Galatians and Romans get high marks. James and Jude (which were not considered apostolic by Eusebius), not so much. Here is his criteria, his true test:
“Now it is the office of a true apostle to preach of the Passion and resurrection and office of Christ, and to lay the foundation for faith in him, as Christ himself says in John 15[:27], “You shall bear witness to me.” All the genuine sacred books agree in this, that all of them preach and inculcate [treiben] Christ. And that is the true test by which to judge all books, when we see whether or not they inculcate Christ. For all the Scriptures show us Christ, Romans 3[:21]; and St. Paul will know nothing but Christ, I Corinthians 2[:2]. Whatever does not teach Christ is not yet apostolic, even though St. Peter or St. Paul does the teaching. Again, whatever preaches Christ would be apostolic, even if Judas, Annas, Pilate, and Herod were doing it. But this James does nothing more than drive to the law and to its works. Besides, he throws things together so chaotically…
—Martin Luther, (LW 35:396-397)
Bishop Michael Rinehart 
October 16, 2014 at 6:07 pm
I am not as smart as Luther was, nor do I comprehend many of the weighty theological issues discussed and debated here, but the book of James is part of God’s word and I am always delighted to read from it. ” Show me your faith, by your works,” James tells us. He points us to Abraham, for example, whose faith was expressed in both word and action. I don’t know why Luther had a problem with the book of James, but who can know the mind of any man except God? James, in the vernacular, is not telling us we are saved by works. He is simply telling us to be doer’s of the word, not just careless do nothing’s spouting about faith without acting on it.