The official King James translation was based, for the most part, on the official Byzantine (Greek orthodox) Bible.
Other translations of the Bible are based on older manuscripts (older copies of the Bible are assumed to be more true to the original text than the newer ones), some of which differ slightly from the Byzantine text here and there.
Some people dislike the newer translations, because they seem to omit certain verses (actually, verses which appear in the Byzantine text but not in the older versions, usually appear in the footnotes -- unless you have bought a really cheap Bible). These people usually point to a verse that appears at the end of the book of Revelations that says that people are not to add or delete words from John's book as proof that other translations of the Bible are corrupt (but how do they know that it isn't the KJV that is corrupt?).
Anyway, it shouldn't matter which translation that you use, as all of the different versions of the Bible seem to say the same thing, despite a slight difference in wording.
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...First, the textual changes in the modern translations affect no major doctrine. The deity of Christ, virgin birth, salvation by grace alone--and all the rest--are still intact. Though certain passages are omitted or changed, the doctrines are not. There are evangelicals who prefer the King James and there are some evangelicals who prefer the modern translations.
Second, the textual changes in these modern translations are based on the most ancient MSS of the Greek NT. These MSS date from early in the second century A.D. But the Greek texts behind the King James belong to a group of MSS--called the Byzantine text--which are much more recent. On the other hand, although these MSS are more recent, they comprise at least 80% of the 5000+ MSS of the NT that we presently have. It is theoretically possible that, at times, these MSS point to an early tradition as well.
Third, the King James NT did not always follow the majority of MSS. Actually, the Greek text behind the King James was based on only about half a dozen MSS. Now it just so happened that these MSS belonged to the Byzantine text. But on a few occasions there were gaps. And the compiler (a man named Erasmus) had to fill in those gaps by translating the Latin NT back into Greek. There are, therefore, some readings in the King James--such as 'book of life' in Rev 22:19 or the wording of I John 5:7-8, which are not found either in the majority of MSS or the most ancient MSS. No serious student of the Bible would call them original (though many popular Bible teachers do).
Fourth, the charge that the more ancient MSS or the men who embrace them are unorthodox is a faulty charge. It is true that in certain places the ancient MSS do not explicitly affirm the deity of Christ--such as in I Tim 3:16. But neither do they deny it! Besides this, in some passages these ancient MSS make Christ's deity explicit where the King James does not! In John 1:18, the modern versions read "the unique one, God" while the King James has "the only begotten Son." Futhermore, the majority of evangelical scholars embrace this critical text. Even the men who edited the New Scofield Reference Bible of the King James Version personally favor the critical text!
Fifth, at the same time, there are some scholars today who are strong advocates of the Byzantine text--most notably, Zane Hodges and Arthur Farstad. Together they edited The Greek New Testament According to the Majority Text and Dr. Farstad was also the senior editor of the New King James Bible. Thus, it is possible to be intelligent and still embrace the Byzantine text, just as it is possible to be evangelical and embrace the modern critical text. (I happen to disagree with the resultant text that Firsthad and Hodges have produced,1 but I respect their scholarship.)
Finally, we ought to quit labeling one another as heretics or idiots in the ongoing discussion. There needs to be charity on both sides. One of my college professors frequently said, "The Christian army is the only army in the world that shoots its wounded!" Unfortunately, this is especially true when it comes to translations of the Bible....