What does Acts 4:18 mean?

Peter and John have broken no laws, either religious or civil. While Jesus and Stephen are, wrongly, charged with disrespecting the temple (Mark 14:57–58; Acts 6:13–14), the guards arrested Peter and John after the two went to the temple for afternoon prayers (Acts 3:1; 4:1–2). But Peter and John teach that Jesus rose from the dead. Not only do most of the members of the Sanhedrin reject the possibility of resurrection, the message of resurrection has accumulated a large following that drive the council mad with jealousy (Acts 5:17).

The Sanhedrin can't refute the resurrection of Jesus because they had no proof (Matthew 28:11–15). They aren't seeking truth, they're seeking to maintain their authority with the people. It isn't a crime to teach the resurrection—the Pharisees do it. It would be a crime to disobey the Jewish supreme court. So they give Peter and John a direct order to stop teaching in Jesus' authority. When they and the other apostles ignore the order, the Sanhedrin has grounds to not only arrest them but to beat them (Acts 5:17–42).

Jesus warned the apostles this would happen. He told them that if they—the unbelievers of the fallen world—persecute Him, they will also persecute His followers (John 15:18–20). He explained, "But all these things they will do to you on account of my name, because they do not know him who sent me" (John 15:21). The Sanhedrin is made up of chief priests, elder-judges of the community, and experts in the Mosaic law. Their job is to enforce the Law that God gave Moses. And yet, Jesus says, they don't even know God.

The apostles, on the other hand, know God. They know Jesus. And they have the Holy Spirit (Acts 2:1–4). And they cannot stop teaching what they know. Their obedience to Christ is worth more to them than their position, their nation, or their lives.