And they were all filled with the Holy Spirit: This event marks when the Holy Spirit came down from heaven. He filled the disciples, as Jesus had promised, to empower them as witnesses of Christ's resurrection (1:4–5, 8). The Spirit's wisdom, energy, and power enabled the church's work and witness (for example, 2:14–41, 43; 4:31; 9:17, 20; 13:9–12; see also Ephesians 5:18).
began to speak in other tongues: The Holy Spirit gave them the ability to communicate so people from other countries could hear about God's works in their own languages (Acts 2:6–11). This passage is one of several references to speaking in "other tongues" (see also 10:44–48; 19:6; 1 Corinthians 14:2–28, 39). This divine gift shows the energy of the presence of God's Spirit. This event marks the beginning of the gospel (the good news about Jesus) being announced to people from every nation.