What does Acts 4:21 mean?
It is a strange dichotomy that God seems to establish His people in ease, then strengthen them in hardship. From the time God called Abraham until Jacob took his family to Egypt, the Israelites saw relative peace and prosperity. Shortly after their arrival in Egypt, the Egyptians enslaved them. In their slavery, they grew from a minor clan to a nation.The early days of the church in Jerusalem are peaceful. For much of the time, the Jesus-followers are not disturbed by the Sanhedrin. But even when the ruling council knows of and disapproves of their message, they are still relatively protected by the good will of the public. After this event, the church grows to about 5000 men, not including women and children, and "multitudes" of converts continued to come—even some priests (Acts 4:4; 5:14; 6:7).
But, like with the Israelites, God uses persecution to make the church really grow. After a mob kills Stephen (Acts 7:54–60), the Sanhedrin commissions Saul to find Jesus-followers and bring them to Jerusalem for trial (Acts 8:1–3). That threat sends the Jesus-followers flying. They fill Judea, Samaria, even up into Cyprus and Syria with the message of repentance and the resurrection of Jesus.
God protects Peter and John from punishment now, but not for long (Acts 5:40). It is distinction from the world, not friendliness with it, which makes the church stronger. That is so, even if the distinction leads to persecution (1 Peter 4:4; 4:12–14; John 15:19).
Acts 4:13–22 covers the reaction of the Sanhedrin to Peter's convicting assertion: that he and John healed a lame man by the power of Jesus' name. The Sanhedrin is frustrated to learn the followers of Jesus—the man they had killed—are in Jerusalem, healing and preaching and gathering more followers. The Sanhedrin wants them out of the way before they grow too popular. So they start slowly by forbidding Peter and John to teach about Jesus. It's an apparent win-win: either these uneducated commoners will stop telling everyone about Jesus or they will disobey a direct order and be vulnerable to greater punishment.
Acts 4 continues the story started in Acts 3. Peter and John have healed a man born lame and preached that Jesus has risen from the dead. The Sanhedrin orders their arrest for teaching the resurrection. The Jewish officials warn Peter and John to stop speaking in Jesus' name. Peter and John refuse, but, since they have committed no crime, the Sanhedrin releases them. Peter and John return to their friends, and the Jesus-followers pray for boldness in the face of growing persecution. The church continues to grow, sharing all their possessions so that no one is in need.