Why is Acts 8:37 missing in the NIV?

Acts 8:27 and a few other verses are not contained in the NIV, NLT, ESV, ERV, ASV, or the latest NASB, because they are missing from the "Codex Alexandrinex" parchment copy of the Greek New Testament, which was discovered within a sealed jar under ~20 feet of sea-water, during the discovery and subsequent excavation of an Alexandrian library in 1881. There are at least two schools of thought among Greek New Testament scholars, regarding which copies are "closest" to the original manuscripts: A) the Alexandrian copy with the least number of copies, or B) the Byzantine copies,with the largest number of documented copy-corrections. The Alexandrian "camp" argues fewer copies means fewer copy-errors. The Byzantine "camp" argues more documented error-corrections means closer to the original manuscripts. I personally find the Byzantine argument more persuasive, because the "missing text" in Act 8:27, looks "missing" when comparing versions. Meanwhile, the arguments are likely to continue.

Specifically, in regards to Acts 8:37, one thing to consider is that there was no stenographer following them around detailing every conversation. These messages were passed along orally for many years before someone compiled them in written form.

Another comment used Proverbs for illustration. We cannot assume that the NIV (and most other translations) gets Proverbs 18:24 wrong and the KJV is correct. As someone pointed out, the KJV was very limited in the number of source documents used. In fact, for the OT the KJV relied pretty much on paraphrasing one Hebrew to English translation, the Hebrew Rabbinic Bible translated by Daniel Bomberg in the 1500s.

Looking specifically at the original language for Proverbs 18:24, the original Hebrew reads "a man of friends will come to destruction." Not only is this a more accurate translation than the KJV, it makes more sense than saying "if you want to have friends you must be friendly." It is a warning to pick your friends wisely.

The KJV has served a great purpose in spreading the gospel, but there is no reason to disparage other translations because none of them are exact. In the same chapter as the question, read Acts 8:32-33 and then read Isaiah 53:7-8 (the verse it is quoting). Even then, translations were not precise and were based on intention and, often, on the views of the person copying it down. This is why most versions include things in the footnotes about translational issues. It's not some nefarious scheme, it's a problem any time you are translating between two very different languages. You have to consider word meaning and intention.

I agree with pretty much everything Jas3.1 said, but let me add a couple of comments:

1. It is certainly true that the manuscripts we have are copies of copies and that some number of discrepancies have crept in through the process. But as I note in my answer to this question: Is it possible for The Bible to contain errors?, most of these discrepancies are trivial spelling errors and the like. None affects any important doctrine.

2. In my copy of the NIV, the verses they omit because they consider them debateable are still included in footnotes. (In other cases they include the verse in the text but then have a footnote saying it is debateable.) Some of my KJV-only friends take the NIV to task to for this. Not to start a fight, but I think it's fair to highlight where there is some debate about the text.

I suppose I can see that this could be construed as a maneuver by someone who doesn't like what the Bible says on a certain subject to try to cast doubt on the doctrine by casting doubt on the reliability of the text. But I don't see any evidence that the NIV has done this. Even without studying the manuscript evidence, there is no pattern to the elisions based on the doctrinal content. If a translation cast doubt on every verse that relates to, say. the Trinity, that would make me suspicious. But I don't see that.