Some KJV-onlyists go so far as to insist that people who do not use the King James Version (or even a specific edition of the King James Version) are not saved. In doing so they believe and teach a heresy — one that violates the Biblical doctrine of salvation by adding conditions not taught in Scripture. [See: Essential doctrines of the Christian faith] Those KJV-Onlyists who teach this in so doing place themselves outside the boundaries of the Christian faith, and should be considered heretics.
Many in the King James Only movement insist that there are conspiracies behind new Bible translations, meant to take away, diminish or introduce various doctrines.
Daniel B. Wallace, Executive Director for the Center for the Study of New Testament Manuscripts, responds:
So, is there a conspiracy today? My answer may surprise the reader: yes, I believe there is. But the conspiracy has not produced these modern translations. Rather, I believe that there is a conspiracy to cause division among believers, to deflect our focus from the gospel to petty issues, to elevate an anti-intellectual spirit that does not honor the mind which God has created, and to uphold as the only Holy Bible a translation that, as lucid as it was in its day, four hundred years later makes the gospel seem antiquated and difficult to understand.2 It takes little thought to see who is behind such a conspiracy.
– Source: Daniel B. Wallace  , The Conspiracy Behind the New Bible Translations
Merely expressing a preference for the King James Version over other Bible versions — while not insisting a) that the KJV is the only legitimate (English) translation, and/or b) that only those who use the KJV are saved — is not King James Onlyism.
Many believers prefer the King James Version as a translation, or even just for the beauty of its language. Doing so is fine.
However, fact is that the language of the KJV is archaic and therefore difficult to understand by modern man.
Henry Neufeld, author of the Bible Translations FAQ, explains:
The main problem with the KJV today is that its language is archaic and people do not understand it clearly. People also find it hard to read. Claims that the KJV is actually easier to understand than the modern versions are simply ludicrous. The effort to push people back to the KJV is not a movement in favor of the authority of the Bible, but is rather against it. Use of the KJV will reduce, rather than increase the knowledge of the gospel message.
In addition, while the KJV was an excellent translation based on the knowledge of its time, new manuscripts and new linguistic discoveries have been made since then that have clarified many passages that were unclear. None of this means that the KJV is a bad translation. It does mean that there are many translations that are better for modern readers because they are written in clearly understandable language. They use the best tools available to make God’s word clear.
– Source: Henry Neufeld  , What about the KJV?
His Greek Grammar Beyond the Basics: An Exegetical Syntax of the New Testament (Zondervan, 1996) has become a standard textbook in colleges and seminaries. He is the senior New Testament editor of the NET Bible. Dr. Wallace is also the Executive Director for the Center for the Study of New Testament Manuscripts.