What does Acts 1:16 mean?

After Jesus' ascension, about 120 of His followers wait in Jerusalem for the coming of the Holy Spirit (Acts 1:5, 15). They spend their time praying (Acts 1:14) and worshiping God (Luke 24:52–53). In one of those prayer sessions in an upper room, Peter stands and addresses the group. The Holy Spirit has given him clarity about one issue they must address. Judas Iscariot, one of the original twelve apostles, must be replaced. He betrayed Jesus and is dead by his own hand (Matthew 27:5). The Twelve are now eleven.

This turn of events was not a surprise to Jesus. During the Last Supper, Jesus had warned the disciples that one of them would betray Him. He identified the betrayer by handing him bread (John 13:21–26). Furthermore, Peter points out that David had prophesied the treachery. Long ago he had written, "For it is not an enemy who taunts me—then I could bear it; it is not an adversary who deals insolently with me—then I could hide from him. But it is you, a man, my equal, my companion, my familiar friend" (Psalm 55:12–13).

After Jesus had given Judas the piece of bread, Satan entered into the wayward disciple. Jesus dismissed him, and Judas left the fellowship of the men he'd traveled with for three years (John 13:27–30). He had already arranged to deliver Jesus to the Jewish leaders in exchange for thirty pieces of silver (Matthew 26:14–16). He gathered a crowd of guards and servants of the chief priests and elders. He knew Jesus would take the disciples to the Mount of Olives, where they have been sleeping all week (Luke 21:37). It would be dark, the disciples would be confused, and the Jewish council would finally have their chance to arrest and try Jesus (Matthew 26:47–56).