Jesus" ascension was already in view in Luke 9:51. There Luke presented it as the ultimate goal of Jesus" first advent ministry. Jesus" ascension would have happened even if the Jews had accepted Him as their Messiah. Prophecies of His glorious return to the earth fill the Old Testament. We should not view Jesus" ascension as an afterthought, therefore. It was rather the culmination of Jesus" first advent. Luke is the only New Testament writer who described the Ascension, both in Luke and in Acts. Perhaps he did so to stress the significance of the resurrection. [1][source]
"With the ascension the Gospel reaches its climax. What began in the temple concludes in the temple with praise to God, and the path of Jesus now reaches its goal. The programme has been established for the second volume of Luke"s work in which the church will obey the command of the risen Jesus to take the gospel to all the nations." [2][source]
"In Luke"s mind the Ascension of Christ has two aspects: in the Gospel it is the end of the story of Jesus, in Acts it is the beginning of the story of the Church, which will go on until Christ comes again. Thus for Luke , as Barrett says, "the end of the story of Jesus is the Church, and the story of Jesus is the beginning of the Church"." [3][source]