We have 5500 hand-written manuscripts of part or all of the New Testament in Greek. The earliest of them are from the second century A.D. Scholars estimate that they differ from each other in several hundred thousand places.
Of particular interest is the fact that the story of the adulteress, the one in which Jesus talks about whoever has no sin casting the first stone, does not appear in any manuscripts or in any commentary on the New Testament before about the 12th century A.D. (Bart Ehrman speculates that something like the following occurred: one scribe in reading John saw Jesus saying things about not judging others. He thought, "I know a wonderful story that's been circulating about Jesus that illustrates that point nicely." He then wrote the story of the adulteress in the margin. A second scribe saw that, and thought, "Gee, this guy accidentally left that part out, and then had to write it in the margin. Let me get it in the main line of the text.") In any case, it is nearly certain that the story of the adulteress did not make it into John until after 1000 A.D.
But why should this worry me one way or the other? It is a great story that teaches an important spiritual lesson. Isn't that what is important, rather than whether or not it was in the earliest version of John?