IN the first book, Theophilus, I wrote about all that Jesus did and taught from the beginning until the day when He was taken up to heaven, after giving instructions through the Holy Spirit to the apostles whom He had chosen. After His suffering He presented Himself alive to them by many convincing proofs, appearing to them during forty days and speaking about the Kingdom of God. While staying with them, He ordered them not to leave Jerusalem, but to wait there for the promise of the Father. “This,” He said, “is what you have heard from Me, for John baptized with water, but you will be baptized with the Holy Spirit not many days from now.”
So when they had come together, they asked Him, “Lord, is this the time when You will restore the kingdom to Israel?” Jesus replied, “It is not for you to know the times or periods that the Father has set by His own authority. But you will receive power when the Holy Spirit has come upon you, and you will be My witnesses in Jerusalem, in all Judea and Samaria, and to the ends of the earth.”
"He presented Himself alive to them by many convincing proofs" - not all of these convincing proofs have been written down in the four Gospel accounts, but we can be sure that they were passed down from generation to generation. In fact, not many things known about what Jesus spoke to His disciples for those forty days. We have just a few stories.
"You will be baptized with the Holy Spirit not many days from now" - this was fulfilled on the Feast of Pentecost. The apostles received the Holy Spirit, Who guided them in their preaching, and they passed down the Holy Spirit when baptizing new Christians. Even to our day, we become members of Christ's Body, His holy Church, and receive the Holy Spirit at baptism.
"Is this the time when You will restore the kingdom to Israel?" - as we can see, even after the Resurrection, after witnessing that Christ has indeed come back to life completely, meaning not simply spiritually but bodily as well, even after all of this, the disciples are still confused as to what kind of Kingdom the Lord was bringing upon the earth. They still expected the restoration of Israel's military power and as the kingdom of kingdoms. Jesus does not rebuke them (maybe He was tired of doing it by now?), but simply says that once they receive the Holy Spirit, they'll understand. And they did. The Holy Spirit opened their eyes and they were able to spread the Good News throughout the known civilization with a tremendous zeal - they realized that the Kingdom of God is not some feeble, temporary thing here on earth, but the realest and truest kingdom we can ever imagine - where there are no wars, no pain, no sickness, no sorrow, but all live in the Presence and Communion with God.
May we inherit this Kingdom.
Yours in Christ,