Is it true that 1 john 5 :7 was added later and was not in Greek manuscripts until the 1500s? Please explain this to me. If so why is it in King James Version? If this is the case, has the Bible been subject to changes by human hands?
Yes, this is true. There was an interpolation to I John 5 (an interpolation is a parenthetical comment which is presumably added in the margin which is later incorporated into the text). Actually, the date you quote is not correct. The interpolation came a few centuries earler, in about the tenth century one manuscript contained this addition (there are three that testify on the earth: the Father, the Word and the Holy Spirit), and one manuscript from the thirteenth century has it in the text, but other than that, all direct evidence comes from the 16th century.
Does this mean that the Bible has been subject to change? The answer is NO! Here is why. Although someone put this addition in the margin in what was presumably a sincere attempt to help readers of his manuscript understand 1 John 5:6-7, we know for a fact that it was not in the original. There is no confusion about this, as the evidence is a slam dunk. Here is the bottom line, all of us can know for certain that this was not in the original. Therefore “the Bible” is certain with regard to this. Because we have literally thousands of Greek manuscripts, minor slips of the pen and even the rare occasion when a good-hearted person made the mistake of making a purposeful small change in a manuscript, with all the evidence, we can with great confidence produce the original and, therefore, the original Greek text is known with great certainty. When you read your English Bible, you will find remarks in the footnotes informing you of such issues when they are significant, so you can judge for yourself.
About the King James Version, this is an inferior translation by today’s standards. It is not that the committee who made this translation did a poor job, but because when they did their work, they only had at their disposal about a dozen Greek manuscripts, none of which were older than 1200 AD, along with Erasmus’s scholarly Greek text. These manuscripts included the interpolated passage in 1 John. We now have more than 5000 Greek manuscripts, including hundreds from before the interpolation found entrance into the Greek New Testament. This mistake in the King James Version does not represent a change in the Bible that we read from the original simply because we have the evidence proving it was not in the original.
I taught a class on this topic this Fall, and go into much more detail than I do here on this topic. I suggest you look at the notes and, if you have time, listen to the audio, which is available for free at my web site. Just do a search for the word reliability or go to https://evidenceforchristianity.org/class-series-in-san-diego-the-reliability-and-inspiration-of-the-bible/