What does Acts 10:46 mean?

Peter and six Jewish Jesus-followers from Joppa (Acts 11:12) are in Caesarea Maritima at the home of the Roman centurion Cornelius. Cornelius, who worships the Jewish God, invited Peter after receiving direction from an angel (Acts 10:1–7). Figuring Peter would have something important to say, Cornelius invited all his friends (Acts 10:24, 33).

Peter had barely started telling them about forgiveness through Jesus when the Holy Spirit fell on the Gentiles (Acts 10:34–44). The Jews from Joppa are stunned (Acts 10:45). Peter hasn't even laid hands on the Gentiles or baptized them, but they are obviously indwelt by the Holy Spirit; they can tell because they're speaking in tongues.

We need to be absolutely clear, here. First, "tongues" is not an unknown language between a Jesus-follower and God. It is a real language, spoken by real people, but unknown naturally to the speaker, as described in Acts 2:4.

Second, tongues is not a universal sign of faith in Christ. Even in the book of Acts there aren't that many recorded cases of someone accepting Christ and immediately speaking in tongues. Since the message of Jesus' offer of salvation has spread to other languages, tongues today are very rare. No one should doubt their salvation because they don't speak in tongues. This is a specific situation in which the Holy Spirit needed to prove a point: Gentiles who are not circumcised or baptized, who have not had an apostle lay his hands on them, can receive the Holy Spirit. Salvation is by grace through faith and nothing we do (Ephesians 2:8–9).