What does Acts 1:15 mean?

Jesus-followers in Jerusalem are gathered, waiting for the baptism of the Holy Spirit, as Jesus had instructed them to do (Acts 1:4–5). In the nine days between Jesus' ascension and the coming of the Holy Spirit, the disciples devote themselves to prayer (Acts 1:14) and worship (Luke 24:52–53). It's interesting to note that Jewish tradition says that while ten Jews are needed to establish a synagogue, 120 Jews are needed for a Jewish community.

While they wait, Peter takes his place as leader. He is an interesting showcase of what the Holy Spirit can do with our natural abilities. During Jesus' ministry, Peter often acted as the disciples' spokesman, but his words were rarely tempered by spiritual understanding. At one point, Jesus even told him his words were from Satan (Mark 8:31–33).

Before the ascension, Jesus had breathed on the disciples and given them a temporary measure of the Holy Spirit (John 20:22). We can only assume that Peter's words here are because of that gift. He now stands and considers their present circumstances. Judas Iscariot is dead. He sees the correlation in the Old Testament, specifically Psalm 69:25 and Psalm 109:8 (Acts 1:20). And so, they need to find a replacement.

Jesus singled Peter out for a special purpose, as a kind of figurehead or metaphor for the spreading of the gospel (Matthew 16:18–19). Peter will go on to be a great speaker (Acts 2:14–41; 3:11–26; 4:8–12, 19–20; 10:34–43). And he will be present at the first mass conversions of Samaritans (Acts 8:14–17) and Gentiles (Acts 10:44–45). With the guidance and power of the Holy Spirit, Peter is growing into the role God has planned for him.