What does Acts 8:37 mean?

This verse is not seen in the oldest and most trustworthy manuscripts of the book of Acts. Most modern translations include it in brackets, or with a note indicating as much. The sentence doesn't express anything contrary to the rest of Scripture, but most likely was not part of Luke's original writing.

Philip has explained the gospel to an Ethiopian court official, and the officer has enthusiastically responded, asking to be baptized immediately. In this verse, Philip gives a challenge and the official responds. The format of the challenge and answer may have even been a practice in the early church: a public confession used to verify the person understands. But the text of this verse, itself, does not seem to be original.

If used as a challenge, this practice is similar to the so-called sinner's prayer. No version of this prayer acts as a magic formula—it does not, itself, save anyone. Rather, it's a convenient tool for someone who wants to follow Jesus but doesn't know the terminology. In a similar way, this call-and-response is a succinct way of showing witnesses the person understands what baptism means.

Today, churches usually have a more involved system. The new believer may go to a class to learn the specifics about Jesus, Christianity, and baptism. Typically, only after showing they understand are they baptized.