What does Luke 24:51 mean?

Jesus has explained how the Old Testament prophets told of His suffering. The disciples witnessed this before and while Jesus bled from the cross. Now, the disciples get to witness the other promise: Jesus "enter into his glory" (Luke 24:26–27).

Jesus had explained why He had to return to heaven during the Last Supper (John 14; 16). His return to heaven meant sending the Holy Spirit to indwell believers. The Holy Spirit will "convict the world concerning sin and righteousness and judgment" (John 16:8), "declare…the things that are to come" (John 16:13), and glorify Christ (John 16:14).

Luke later tells Theophilus that the disciples don't immediately leave after Jesus rose into heaven. They gaze into heaven until two men dressed in white robes—angels—give them a similar message to the one given the women who came to the empty tomb. Instead of, "Why do you seek the living among the dead?" (Luke 24:5), they say, "Men of Galilee, why do you stand looking into heaven? This Jesus, who was taken up from you into heaven, will come in the same way as you saw him go into heaven" (Acts 1:11).

This time, they believe. They return to Jerusalem and celebrate in the temple, praising God that the Jewish Messiah has come and will come again (Luke 24:52–53).