From Acts of the Apostles, by William S. Kurz, SJ, commenting on Acts 2:1-13:
This inspiring account of the Church’s inception is a model for contemporary Christians. Christian identity today is likewise grounded in our receiving the Holy Spirit in baptism and confirmation. Christians are sometimes tempted to think they can be saved by their determination to cultivate virtue, by doctrinal orthodoxy, or by scrupulously following rules and commandments. The Pentecost event reminds us how indispensable the Holy Spirit is to our faith and salvation. The indwelling Holy Spirit bestows on us the theological virtues of faith, hope, and love, from which all other Christian virtues and actions follow (Catechism 1812–13). The Spirit produces a particular kind of fruit in us (Gal 5:22–23), the character of Jesus.
The descent of the Spirit at Pentecost transformed the first followers of Jesus, who had previously hidden in fear from those responsible for Jesus’ crucifixion and were unable to understand God’s plan for a crucified Messiah (Luke 9:44–45; John 20:19). After the Holy Spirit’s coming, they understood God’s saving plan and were able to proclaim it boldly, even at the cost of beatings, imprisonment, and martyrdom. The charisms of prophecy and of speaking in tongues, as well as healings and miracles in the name of Jesus, were abundantly evident. The disciples were filled with joy and continual praise, even in the face of persecution. Today the Spirit is likewise needed to empower Christian life and ministry and to make it fruitful in bringing others to salvation. Without the Holy Spirit, there are no Christians. Without the Holy Spirit, there is no Church.