The disciples were focusing on “small” visions for Jesus: restoring a political kingdom. Jesus calls them to focus on the grander vision: being His witnesses to the ends of the earth. He provides them a means to carry out this vision in the Holy Spirit. And when the disciples are caught focusing on Jesus going up to heaven, the angels remind them to focus forward into their future.
Jesus' work on the cross may be finished, but His work in us is just beginning. All of us have doubts about a vision for the future. Jesus reveals God’s grace to cancel our doubts. We must have faith in order to see what is unseen and He will reveal where He would have us go next.