In this instance, being filled with the Spirit refers to being equipped or strengthened to speak God’s Word in the face of threats and opposition.1 With this prayer and God’s answer we see a clear parallel to Pentecost: the Spirit comes in answer to prayer (Acts 1:14; Acts 4:24–30), the Spirit fills all those who were present (Acts 2:4; Acts 4:31), and the Spirit equips the apostles to proclaim the wonders of God (Acts 2:11; Acts 4:31).2
31 And when they had prayed, the place in which they were gathered together was shaken, and they were all filled with the Holy Spirit and continued to speak the word of God with boldness.