The Greek New Testament (GNT) that was used by the translators of the King James Version is today called the "Majority Text" or the "Textus Receptus" (Latin for Received Text, abbreviated as TR). It was compiled by a Dutch scholar and theologian, important during the Renaissance movement, known today by his middle name: Erasmus.
The New Testament documents were written by hand, of course, and passed from person to person, copied and recopied over the centuries. Over time, a number of differences developed between copies. Most of these were spelling variations, such as we see in English with "color" and "colour", or insertions of an "And" or "But" at the beginning of a sentence.
With the growing interest in Scripture as the Reformation unfolded, there was a keen desire to know as closely as possible the original words of the Bible writers.
The copyists who lived from the first century down to the 15th century were often very godly individuals who had memorized long portions of Scripture themselves. Some of the differences between manuscripts seem to have arisen as copyists started writing from memory rather than from the page in front of them...and mixed up one Bible passage with another that was very similar.
An example of this would be in 1 Thess 1:1, where the best manuscripts have "Grace to you and peace." The best manuscripts of 2 Thess. 1:2 have Paul saying "Grace to you and peace from God our Father and the Lord Jesus Christ."
Sometimes when the copyist was writing out the book of 1 Thessalonians, he would, without looking, add the "from God our Father...." phrase from 2 Thessalonians.
It is the job of the believer who takes on, as his mission from God, the task of sitting in a library comparing manuscripts, to be responsible for thinking through the kinds of differences he sees before him, and hypothesize about how those differences could have arisen. Out of that, he then chooses what he judges to be the most likely original wording, and includes it in his GNT edition.
As an aside, Bible translators for the last 250 years have unanimously concluded that there was never any intentional manipulation of Scripture by copyists over the centuries. All the variants appear to be honest mistakes, and not a result of trying to grind some theological axe. We owe a lot to these nameless copyists over the centuries...and to the (relatively) nameless translators and textual specialists who have collated the various manuscripts from all these copyists.
Erasmus sought to address this urge to know the precise words of Jesus and Paul by compiling his own edition of the GNT. He started by collecting all of the NT Greek manuscripts he could (i.e. copies of copies of copies, etc. of the originals - none of the original manuscripts have survived) and collating all the differences between them. Where there were differences, he counted the manuscripts on his desk to see what the majority of them included. He looked to the tallest stack of manuscripts that used a particular reading, and included that reading in his edition.
Hence the name Majority Text. This is the version of the text that appears in the majority of manuscripts.
However, there was an enormously high regard for the medieval Latin translation of the Bible, called the Vulgate. There were times that the Vulgate did not seem to be translating the same Greek as Erasmus had on his desk. When this happened, he would reverse-engineer the Latin into Greek. So he included not only the text from various Greek manuscripts in his version, but also included his own hypothetical Greek phrases back-translated from the Vulgate.
The translators of the King James Version (1611) used Erasmus’ edition of the GNT as the basis of their translation. Martin Luther used it as the basis of his translation of the Bible into German (1522).
Brooke Foss Westcott and Fenton John Anthony Hort were a pair of English believers who had the same interest as Erasmus - in knowing the words of Jesus and Paul as exactly as possible - but who went about their task differently.
They said that if you have dozens - hundreds - of manuscripts
from AD 1300, and six or twelve manuscripts from pre-AD 400, you
should probably give more weight to the older manuscripts, even
though there are fewer of them. Their idea was that there
was a good deal less time for copyists to make spelling errors
when there were only 200 to 400 years that elapsed from the
writing of the originals.
We have one papyrus copy of the NT, known as "p46", that is our best source for the letters of Paul. p46 was copied perhaps as little as 100 years after Paul wrote his letters. So we don't have any of Paul's original writings. We don't even have any of the first copies of Paul's letters (and they were all copied almost immediately and circulated to other churches in their areas). But with p46, we may have a copy of a copy of Paul's original letters.
In contrast, the oldest manuscript we have of Julius Caesar's account of his campaigns in Gaul was copied 1,000 years after the original was penned. The quality of our biblical manuscripts is astounding compared to every other ancient document. There is much more that could be said about this, but we need to get back to Westcott and Hort here.
They gathered together the oldest manuscripts they could lay their hands on and collated them with "the majority text" of Erasmus. Where there were differences, Westcott and Hort almost always decided that the older a manuscript was, the more reliable it was likely to be.
They leaned heavily on the Codex Vaticanus, so named because it was discovered in the Vatican Library. Codex Vaticanus (abbreviated as "B") dates from the A.D. 325-ish.
The other manuscript they made heavy use of was the Codex Sinaiticus, so named because it was discovered (in 1859) in a monastery in the Sinai Peninsula by von Tischendorf. Siniaticus (abbreviated as the Hebrew character aleph: "א") also dates from A.D. 325-ish
Although we owe a lot to Westcott for his scholarship, and indeed he spent an enormous amount of time in his office surrounded by dusty manuscripts, during his own lifetime he was also known for his practical interest in social justice and missions. Four of his sons left to be missionaries in India, and of his work while serving as the Bishop of Durham Wikipedia says:
Upon one famous occasion in 1892 he succeeded in bringing to a peaceful solution a long and bitter strike which had divided the masters and men in the Durham (coal mines); and his success was due to the confidence which he inspired by the extraordinary moral energy of his strangely "prophetic" personality, at once thoughtful, vehement and affectionate.
Almost every Bible translator today has been totally convinced that Westcott and Hort "got it right." Their translations are all based, with just a few tweaks from manuscripts that were uncovered in the 20th century, on the Westcott and Hort edition of 1881 (abbreviated WH), including:
- New International Version
- New American Standard Version
- Good News Version
- Revised Standard Version
- English Standard Version
- NET Bible
It is not just English versions that have used WH. Missionaries with Wycliffe Bible Translators have been translating the Bible around the world for the past 80 years, into hundreds of languages. All of their versions are based on WH.
So while newer translations do replace the "thee" and "thine" of the KJV with "you" and "yours", that is only one of the differences. A more fundamental difference is that they all make use of the WH edition of the New Testament...which is based on dramatically older and more accurate manuscripts.
The one recent translation that has achieved reasonable circulation, and continues to be based on the Majority Text (a.k.a. Textus Receptus, or "TR"), is the New King James Version. The NKJV updates instances of "thee" and "thou", just as do all modern translations. Its distinctive is that it continues to be based on the TR.
The translators of the NKJV allege that recent manuscripts, copied out late in the Dark Ages period, are intrinsically more likely to be accurate than ones made a thousand years earlier, at the beginnings of the Christian era. Most thoughtful Christians just think the NKJV translators have their heads stuck in their...sentimental attachment to the King James Version.
The two most obvious differences between the TR and WH are that WH omits John 8:1-11 and Mark 16:9-20.
There are some churches in the American southeast that have built their worship services around the handling of poisonous snakes, a la Mark 16:18. They, of course, find the idea unthinkable that the WH text might be closer to what Mark actually wrote than the TR.