The Apostles, filled with Holy Spirit, began faithfully bearing witness in Jerusalem that by His death and resurrection Jesus Christ was proven Lord and Savior. Their witness had a multiplying effect.
The people they told about Jesus didn’t see Jesus raised from the dead or ascend into the clouds, but they too became witnesses of Jesus’s Lordship and saving work. By the power of the Holy Spirit, people are still being lead to Jesus today who then lead others to Jesus. Disciples are still making disciples. The Kingdom of God is advancing, and the good news of Jesus is calling us to be His witnesses.