There are now over 8000 Greek manuscripts available to us. The Codex Vaticanus, Codex Alexandrinus and Codex Sinaiticus are entire Greek New Testaments from about AD 350. There are also dozens of manuscripts from the third and even the second century AD. Given all this data, scholars are able to make very solid conclusions about nearly every significant question about the Greek text. If scribes were to change the text for whatever reason, perhaps to make the original more clear or possibly even to add their own theological interpretation, this change would show up by comparison to thousands of other manuscripts. I will admit that we do not have a perfect manuscript of the entire New Testament in Greek, but the number of significant textual uncertainties is very low. You can be quite confident that virtually all the Greek text is the same as the original. There are a few passages with some doubt. Famous examples are Mark 11:26, Acts 8:37, 1 John 5:7-8 and a couple of others. Even in these cases, based on the evidence, we can be very confident about what was the original text. This is a very limited asnwer. A lot more can be said. One resource is my book Reasons for Belief (www.ipibooks.com).