And now, Lord, look upon their threats and grant to your servants to continue to speak your word with all boldness,
This is a prayer of commitment to the gospel despite heavy opposition to the gospel. Peter and John were arrested by the temple guard, held over night, and brought before the Sanhedrin. This was the same group that had arrested, beaten, and handed Jesus over to Pilate for crucifixion. And from their very first questions to Peter, the gospel is boldly proclaimed in the apostles' answers (Acts 4:10-12).
This boldness is so pronounced that the Sanhedrin takes note of how unusual it was for "uneducated" men to be so well prepared. They recognized the stamp of Jesus in them (Acts 4:13), for Jesus too had taught with "all authority and not as the teachers of the law." Flustered by the clear miraculous healing that had taken place in the temple, and befuddled by the bold nature of the teaching, the council commands the two apostles to cease and desist all their teaching about Jesus (Acts 4:18). They hope to intimidate them. It is a futile effort.
Powerfully, the goodness of the gospel only grows as, into the very faces of their accusers, Peter and John politely decline the gag order (Acts 4:19-20). And in the awkwardness of the acclaim of the people who were turning massively to belief in Christ, the officials are left with only threats. The apostles are released to boldly continue in their mission.
The growing church convenes immediately in praise and prayer, confident in the call to preach Jesus to everyone. They knew they would be opposed, but believe God's sovereign purposes to be much greater than the purposes of their enemies (Acts 4:27-28). And it is that conviction that fuels their boldness with the gospel. They knew God would continue to honor their preaching of the gospel as they did their ministry in the name of Jesus.
There were two commitments to their boldness. First, they were committed to Jesus' command to preach the gospel (Acts 4:20). Secondly, they were committed to trusting God's sovereign control as they preached it (Acts 4:28). And in that firm conviction, boldness poured out in the gospel message.