There are a large number differences between the KJV and the NRSV, RSV, NIV, NASB, etc. Many of these differences are quite minor, reflecting only the changes which have occurred in the English language over the past four centuries. There are, however, numerous differences which are due to copy errors in the parent-manuscripts which are behind our many various versions. The New Testament Scriptures were copied, by hand, for one thousand five hundred years . . . copy-errors were bound to creep into the text along the way! Most of these additions and subtractions were made by mistake, but sometimes they were intentional. We are lucky in that we have a large number of manuscript copies of the Scriptures, and can cross-check any particular copy against all the others and there-by find most of the changes, most of the additions and subtractions, which have drifted in. The vast majority of the changes and errors which have made their way into the manuscript record are easily filtered out by this cross-comparison of the many various manuscripts. The process of filtering out changes has been going on for a long time, and while we are blessed with a large number of manuscripts with which to work, we are also extremely fortunate to have some very ancient copies of the Scriptures as well.
Most of the modern translations of the New Testament were made from copies which date anywhere from 75 to 150 years after the originals. Subsequently, our oldest copies are so close to the originals that the chances of any major errors having crept in are significantly reduced. Such hasnt always been the case, however. Its only been in the last 100 years that we have had copies that got us to within 500 years of the originals, and only since 1952 have we had access to copies of the New Testament which get us as close to the originals as the life span of a human being.
As I have already said, the KJVs New Testament was produced from a critical edition of the Greek New Testament which was based upon 12 manuscripts, none of which were older than 1000 AD, and one has been dated to 1536 AD. While it is true that the vast majority of the Scriptures have remained, miraculously, unchanged, there are a number of significant words and, even, whole verses which crept into the Scriptural account that were not there in the originals. This is not theory or speculation ... it is an established fact.
Let me give you a few examples of what I am talking about. In the KJV, Acts 8:37 says the following:
And Philip said if thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God.
If you will check almost any modern translation, however, you will notice that the entire verse is missing from the text. The NRSV goes straight from verse 36 to verse 38, and the only way one would know that there is a difference (besides versification and, perhaps, memory) is that the editors of the NRSV have supplied a marginal note which informs the reader that "other ancient authorities add...." verse 37.
Why has verse 37 been left out of the text of the Acts of the Apostles in most modern translations? The answer is very simple: there are absolutely no examples of Acts 8:37 to be found in any copy of the New Testament prior to the year 550 AD. Our oldest copy of The Acts of the Apostles dates from around the 180s AD, and it lacks these words. Verse 37 is also missing from many other copies, some dating to as late as 1350 AD. This can only mean one thing: verse 37 is an addition, one which is probably dependent upon the Baptismal liturgies of the time.
Another good example of a verse that can be found in the KJV but which is missing from almost every other English translation of the Bible is 1 John 5:7/8
For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost; and these three are one.
The NRSV has only the words: "There are three that testify:...." and then goes straight into verse 8. Why would such an extremely important affirmation of the doctrine of the Holy Trinity be lacking in our modern versions? Well, it’s certainly not because we reject the doctrine! Scriptural support for the doctrine of the Holy Trinity can be found in multiple locations throughout the Scriptures. We are not dependent upon any one verse to support this core doctrine of our faith. No, these words have been removed for one simple reason: not a single copy of the Greek New Testament, from the oldest up until the early 1500s have the verse! Yes, that is correct, it is missing from the body-text of every ancient Greek manuscript of the New Testament. The oldest Greek manuscript with the verse in its body-text dates from 1536 AD ... and shows every sign of having been translated into Greek from Latin. Additionally, there are several dozen Greek Manuscripts which have the verse written in the margin, sometimes translated from Latin and sometimes in Latin!
Now, this doesnt mean that the verse was made up in the 1500s, or even that what it says isnt true. Quite the contrary, some Latin Bibles from as early as 500 AD have the verse. Additionally, several Church Fathers, starting in the 700s, quote the passage in Latin. However, the originals of the New Testament were not written in Latin, but in Greek, and if we dont have any examples of the verse in the Greek text the New Testament, how can we possibly say that it was in the original?
The verse does convey the doctrine of the Holy Trinity as we believe it; but, it isnt from the hand of St. John. It probably found its way into the the text sometime in the fifth or sixth century as a marginal notation in a Latin copy of the New Testament.