The key point of Luther's biblical exegesis was his conviction of Christ's being the rex scripturae.1 There is a famous passage from his preface to the "Epistel S. Jacobi und Judas" saying:
This is the right touchstone to criticize all the books: See if they preach Christ or not. […] What Christ did not teach, that is not apostolic, may it have been taught by St. Peter or Paul. And what Christ did teach, that is apostolic, may it have been taught by Judas, Annas, Pilate or Herode.2
With this key exegetical principle, that the doctrine of justification is paramount,3 Luther found a critical standard for the individual biblical writings. The Epistle of James,4 Second Maccabees, Revelation, and Esther did not meet that standard in his eyes. He was so hostile to them that he wished they didn't even exist: he wanted to "almost put them out of the bible."5
1: 40 I, 421
2: "Auch ist das der rechte prufsteyn, alle bucher zu tadelln, wenn man sihet, ob sie Christum treyben odder nit. […] Was Christum nicht lehret, das ist noch nicht apostolisch, wenns gleich S. Petrus oder Paulus lehret. Wiederum, was Christum prediget, das wäre aposolisch, wenns gleich Judas, Hannas, Pilatus oder Herodes tät", WADB 7, 384
3: And mainly the things that are taught in the Gospel of John, the Epistle to the Romans, and the First Epistle of Peter.
4: There's a dinner speech where he once said: "I'm going to heat up the oven with Jeckel [=Jakob=James]." ("Ich werdem einmal mit dem Jeckel den Ofen heizen."), Ti. 5,382,17
5: "schier aus der Bibel stoßen", Ti. 5,414