Most people are aware that Luther added the word "alone" to Romans 3:28 in his German translation, but few people are aware of an equally pernicious attempt to mess with God's Word done by John Calvin when he inserted the word "alone" into James 2:24. In reality, the word "alone" does not appear in the Greek of James 2:24. If one is going to be faithful to Scripture, a Catholic can no longer say that James opposes "justification by faith alone," because James never speaks of "faith alone" in the first place. But there's good news about this, because once we see why Protestants have continued to follow Calvin by adding the word "alone" to James 2:24, we will be able to refute Luther's heresy all the more easily.
Luther demonized the book of James because Luther rightly saw that James' Epistle was incompatible with Luther's interpretation of Paul's Epistles to the Romans and Galatians. He rightly figured that something had to give, and it made more sense to dispense with James' one Epistle than with all of Paul's writings. John Calvin would likely have rejected James as well, but he realized that something more important was at stake: the integrity of the canon of Scripture. Calvin rightly recognized that if Protestants threw out James, then it would be a free-for-all with the canon, which would demolish Sola Scriptura. So Calvin came up with an last ditch effort by adding the word "alone" to James 2:24 and thus (temporarily) saving both key doctrines of Scripture Alone and Justification by Faith Alone.