Never thought about the Ascension of Jesus being vital to the Gospel. Tim Chester & Jonny Woodrow show how it is indeed wonderful news in their new book. They say,
Jesus ascended into heaven ‘to appear for us in God’s presence’. Atonement was not complete until Jesus stood before God on our behalf. ‘If he were on earth, he would not be a priest, for there are already priests who offer the gifts prescribed by the law’ [Heb. 8:4]. …
Jesus, our priest and mediator, appears in the presence of God, bearing our names, as a memorial to God. He is the sign, the reminder, the pledge, the guarantee that we belong in the presence of God. Our presence before God is as certain as Christ’s presence before God. Our salvation is safe and secure as long as Christ is in heaven.
This is why the ascension is such good news. This is one reason why we would not want Jesus to leave heaven and appear on earth, however convenient that might appear for world evangelism. If Jesus were to leave heaven then the pledge of our salvation would be removed. If Christ is not in God’s presence on our behalf then we are not in God’s presence.
Quoted from Tim Chester & Jonny Woodrow, Ascension: Humanity in the Presence of God, pp. 15-16.