What does Acts 5:18 mean?

Scholars debate whether the priests arrest all twelve apostles or if this is a second phase of punishment following the event in Acts 4:1–22 when Peter and John were taken in. The narrative highly suggests this second event involves all the apostles. The council initially held Peter and John because they taught the resurrection from the dead, which the Sadducee-heavy Sanhedrin doesn't believe in. Since preaching the resurrection isn't a crime—the Pharisees do just that—and the public supported them too much, the council couldn't punish them.

Now, however, the Sanhedrin can charge the apostles with disobeying a direct order of religious authorities. The Sanhedrin had forbidden Peter and John from preaching in Jesus' name (Acts 4:18). The apostles disobeyed, as Peter warned they would (Acts 4:19–20).

This is not the last time Peter will see the inside of a Jerusalem jail cell. After a mob murders Stephen (Acts 7:54–60) and Saul drives the Jesus-followers out of Jerusalem (Acts 8:1–3), Herod Agrippa I will arrest both Peter and James, John's brother. Herod will kill James; Peter will escape with the help of an angel (Acts 12:1–7).

As a literal term, "apostle" refers to someone sent out as a messenger. Mary Magdalene was this generic kind of "apostle" when Jesus sent her to the Eleven to tell them about His resurrection (John 20:11–18). Paul is an apostle to the Gentiles (Acts 13:2–4; Romans 11:13). But when the word is used in the very early church in Jerusalem, it refers specifically to the eleven remaining disciples of Jesus and Matthias whom God chose to replace Judas (Acts 1:26). As an office, or a title, it only applies to those men.