Within the broad camp of "King James Onlyism" are a variety of groups and perspectives that do not always agree with one another on the details. Indeed, many King James Onlyists will claim to not really be King James Onlyists at all! Instead, they say that it is not so much the King James Version that one must use but rather the specific Greek and Hebrew texts behind the KJV. They claim that their problem with modern translations is purely in their use of later critical Greek New Testaments rather than the traditional "Textus Receptus" or "TR" used by old translations like the KJV (special attention is generally given to the Greek New Testament as the Hebrew texts used by historical and modern translators are virtually identical).
This seems like a simple distinction: Some are claiming the English Translation itself is the ultimate standard while others are claiming that the Greek text behind it is. Some, thus, are truly KJV Only whereas others are actually TR Only, preferring the KJV purely on its faithfulness to that traditional Greek text. The problem is that, in practice, the line is often not so hard and clear. Many people who argue as if they were (and may genuinely think of themselves as) TR Only prove to be in practice, when push comes to shove, King James Onlyists whose trust is in the Elizabethan English translation and not the Greek TR. Here are a few good ways to test whether someone is, at heart, a true TR Onlyist or if they are a KJV Onlyist who depends on TR Only arguments to shield their position.
Modern Translations of the TR
If the issue is not the KJV itself, if it is really just about using the TR rather than the modern critical Greek texts, then the person should be open to other translations from the TR. Ask them how they feel about the New King James Version (NKJV) or the Modern English Version (MEV)? Both of these are modern English translations that utilize the TR rather than relying on modern textual criticism. The MEV does not even include textual footnotes in passages where manuscripts vary. If the person is completely comfortable with someone using such translations or says that they have never looked into them but would be very open to it, they are probably a genuine "TR Only" advocate. Their loyalty is not to the KJV alone but rather to the underlying Greek text.
If, however, they denounce the NKJV and MEV right alongside all other modern translations or produce a litany of excuses and caveats as to why those translations of the TR can't be trusted and (either overtly stated or implied by implication) only the KJV's translation of the TR should be trusted, then the person is not really TR Only. At heart, they are King James Only. It is the English, not the Greek, to which they are bound. Indeed, many KJVOs also go after older translations from the TR, such as the Geneva Bible, because in some places they use different English words than the KJV does. This is a clear sign that the person is not really defending the TR at all. It is the KJV and the KJV alone in which they are interested.
Better Translations of the TR?
If the issue is really about the TR and not the King James English, then the person should be open to the idea that other translations of the TR may, in some places, be better or clearer than the classic language of the 1611. Take, for example, 2 Peter 1:1. The King James reads:
"Simon Peter, a servant and an apostle of Jesus Christ, to them that have obtained like precious faith with us through the righteousness of God and our Saviour Jesus Christ," (2 Peter 1:1, KJV).
While not "wrong," this translation is unclear. The language allows one to read "God" and "our savior Jesus Christ" as two different people. Modern translators of the TR, however, render the text more directly:
"Simon Peter, a bondservant and apostle of Jesus Christ, To those who have obtained like precious faith with us by the righteousness of our God and Savior Jesus Christ," (2 Peter 1:1, NKJV, see also the MEV).
Here, there is no room for misreading. Jesus Christ is both God and Savior. This is not a novel rendering of this verse. Translators of the TR even before the KJV read "our God and Savior Jesus Christ." See, for example, the William Tyndale New Testament, the Geneva Bible, or the previous "authorized" versions: the Great Bible and the Bishop's Bible. Devout translators of the TR have agreed for hundreds of years that this is the correct translation, the KJV being the only major exception. So, if someone claims that they are not KJV Only and are instead TR only, show them this and ask if they would agree that "our God and Savior Jesus Christ" is a better translation than "God and our Saviour Jesus Christ." If they concede this, admitting that the KJV is not the best translation of this verse, they are really TR only. If they are unwilling to concede even here that the KJV could be inferior to other translations, you are talking to a King James Onlyist. They are not interested in what the TR actually says. They are defending the TR as a means to defend the King James. They are devoted only to a particular English translation.
There are real TR Onlyists, and it is worth walking through the issues with them and working through textual differences in a civil and brotherly manner. That said, I would generally just encourage them to, in addition to the KJV, read also from the NKJV and/or MEV to help them better understand the text. While I do not think the TR is the best Greek New Testament text available, the differences are not really that significant and no central Christian doctrine hangs on any textual variant. There are fine translations that come from the TR and there is no harm in faithful Christians relying on them.
Many King James Onlyists, however, wish to use manuscript differences and textual variants to turn the argument away from the real issue with King James Onlyism: the changing English language. By making the discussion about the TR rather than about the wording of the KJV itself, they force people who don't read ancient Greek to argue about the ancient Greek, an argument that it is difficult to win. They thus shield their position. The biggest issue with King James Onlyism is that, even if the TR was the best preserved Greek text, the KJV would still not be the best translation for today's readers to choose, and for two reasons. First, the English language has changed so that the words, phrases, idioms, syntax, and even punctuation found in the KJV no longer mean to us today what they did to readers in 1611. Wording that was perfectly correct in the 17th century no longer conveys the correct sense of the passage in the 21st. Four hundred years later, we speak differently and need translations in our own English.
Secondly, our knowledge of ancient Greek and Hebrew has increased since the KJV was produced. In 1611, the Western world was still rediscovering the Biblical languages after a thousand years of knowing the Bible only in Latin. Translators then did incredible work with the knowledge they had, but even they knew that their expertise in these ancient tongues was wanting, which is why there are so many Reformation-era English translations. They kept trying to improve them, not as a repudiation of each other, but to build on each other in the constant quest for the most accurate translation possible. None of them (the KJV translators included) was ever audacious enough to say, "okay, that's it, we've made it. No human being could ever do any better than the work we have done right here." Thus, even if the TR is the best possible Greek text, it still behooves us to apply all we have learned about the Greek language since 1611 and try to make sure we have the best translation of the TR possible. The TR Onlyist can agree with this, but the King James Onlyist cannot. That is where the real issue lies.
If the point of translation is to make the Bible comprehensible to the average English-speaking Christian, it makes no sense to hold people back from using the clearest English for today's readers and the best scholarship on the Greek language to produce the most understandable and accurate translation possible. The King James Onlyist must, by definition, oppose this and thus stand against the only good reason to translate the Scriptures at all. Until that is reconciled, there is little point arguing over variant readings.