What does John 5:4 mean?

This verse is not typically found in modern translations. Those which include it often note that it is not found in older or more reliable manuscripts. The disputed text indicates that an angel would agitate the water of the pool, and the first invalid into the water after that would be healed. This raises theological questions, such as why God would make healing into a contest, or express miraculous power in such a random yet repetitive way.

According to manuscript evidence, the most likely answer is that these words were not part of the original text of the gospel of John. They might have been written in a margin, as a note, or a traditional explanation of the pool. At some point, the note might have been inadvertently copied into the main body and absorbed as part of the text.

Ultimately, this is a better example of biblical reliability than of biblical weakness. Verses such as this represent the most extreme version of "variants" found in scriptural manuscripts. Accepted or not, the text has no bearing on any doctrine, belief, or other meaningful facts of the faith. In other words, its exclusion isn't important, and if it were included, it wouldn't overwhelm the rest of what the Bible says on related topics. The Bible is an amazingly well-preserved and well-documented work.