"You will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth." (Acts 1:8)
The Acts of the Apostles is the second volume of Luke's two-part work, addressed again to Theophilus and opening with a direct reference to the Gospel: in my former book I wrote about all that Jesus began to do and to teach. The word began is significant. The Gospel records what Jesus began. Acts records what he continues, through the Holy Spirit working in his body the Church. The risen Christ is the subject of both books. He does not withdraw at the Ascension; he acts through those he has sent. The Catechism calls Acts the Gospel of the Holy Spirit, the continuation of the mission of Christ through the community of the baptised (CCC 735).
For forty days after the Resurrection, Jesus appears to the apostles and speaks to them about the Kingdom of God. The number forty, like the forty days of the desert and the forty years of the wilderness, signals a period of formation and preparation. They are being prepared for what is coming, though they do not yet know what it will be. When they ask whether he is about to restore the kingdom to Israel, he redirects them: times and seasons are the Father's business. Yours is witness.
Jesus is taken up before their eyes and a cloud hides him from their sight. Two men in white stand beside the stunned disciples: Why do you stand here looking into the sky? This same Jesus, who has been taken from you into heaven, will come back in the same way you have seen him go into heaven. The Ascension is not an abandonment. It is a change of mode: the localised presence of the carpenter from Nazareth becomes the universal presence of the glorified Lord, present everywhere the Spirit goes. The Catechism describes the Ascension as Christ's entry into the heavenly sanctuary, where he intercedes for us forever at the right hand of the Father (CCC 665).
The community returns to Jerusalem and Peter leads the selection of a replacement for Judas, citing Psalm 69 and Psalm 109 as Scripture that pointed to this vacancy. Two candidates are put forward; lots are cast; Matthias is added to the eleven. The reconstitution of the Twelve to their full number before Pentecost is theologically deliberate: the Church at her founding must have the full apostolic college, the twelve foundations of the new Jerusalem (Revelation 21:14), in place when the Spirit falls.
Brothers and sisters, the programme of Acts 1:8 is the programme of the Church in every age: Jerusalem first, then Judea, then Samaria, then the ends of the earth. Concentric circles expanding outward from where you are. You cannot skip to the ends of the earth without starting in Jerusalem, the place where you actually live, among the people you actually know. Begin there. The Spirit will take it from there.
Lord Jesus, you ascended in glory and promised the Spirit to make us your witnesses to the ends of the earth. Come, Holy Spirit. Make us witnesses: in this city, in this neighbourhood, in this family, beginning today. Through Christ our Lord. Amen.