... [T]here was an internal necessity for Christ’s work in His in-humanization to be completed. The Lord could not remain on earth longer than that period of forty days during which were accomplished the mystically definitive glorification of His resurrected Body and the preparation of this Body for Ascension. Just as He had prepared Himself for forty days in the desert for His earthly ministry, so forty days also passed before His Ascension. As long as the Lord remained on earth, the work of salvation and the deification of the human essence were not yet complete.
The Lord came “in order to unite earth with heaven” (according to the kontakion). That is why He came down from heaven and became incarnate from the Virgin Mary, while being true God, “one of the Holy Trinity.” He was also to follow the reverse path, this time not of descent, but of Ascension. The heavens, which opened for His descent down to creatures, also had to open for the Ascension of the Incarnate One. If the Lord had remained on earth and had not returned to heaven, that would have signified that there is no road for the human essence to heaven, and that the One who assumed this essence would have been separated from the Holy Trinity, would have remained powerless, as it were, to deify and glorify the human essence....
The Ascension to heaven and the sitting at the right hand of the Father signify the total and definitive salvation and deification of the human essence: There is no separation between the heavenly and the earthly, for, in heaven, God who bears human flesh sits on the throne.... Christ’s Ascension not only completes His redemptive work; it also opens the way for the descent of the Comforter Spirit into the world, for the establishment of Christ’s Church on earth. The Ascension is the foundation and preparation for Pentecost: “The Lord ascended to heaven in order to send the Comforter to the world” (according to the sticheron).
Sergius Bulgakov, Churchly Joy: Orthodox Devotions for the Church Year, trans. Boris Jakim (Grand Rapids, MI; Cambridge, U.K.: William B. Eerdmans Publishing Company, 2008), 122–123.