Jesus used to appear to the apostles for forty days. At the end of the forty days he asked them not to move until they get the Holy Spirit. Acts : But 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." Jesus was taken to heaven right before their eyes. They needed one to replace Judas, and Matthias was chosen