Then will appear in heaven the sign of the Son of Man, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory (Mt 24:30).
Behold, he is coming with the clouds, and every eye will see him (Rev 1:7).
i) How will Jesus return? Some people think the depiction of Jesus literally coming down from the sky reflects an antiquated mythological cosmography, where heaven is "up".
ii) In theory, this could be stock imagery without being mythological. We need to distinguish between mythology and dead metaphors.
iii) In addition, there's the question of how Christians should update futuristic descriptions. The Bible uses period imagery when depicting the future. Imagery that reflects the world familiar to the original audience. But if this is really about the distant future, then we need to make some mental adjustments.
iv) In theory, Jesus could return the way he suddenly appears to people after the Resurrection. There he appears out of nowhere. He appears and disappears out of thin air. That doesn't require Jesus to come down from the sky. And this also shows that Jesus reappearing isn't necessarily wedded to an allegedly obsolete cosmography. Jesus needn't pass through space to appear to someone. At least not visibly.
v) That said, a basic problem with dismissing the depiction of Jesus coming down from the sky is the Ascension account.
9 And when he had said these things, as they were looking on, he was lifted up, and a cloud took him out of their sight. 10 And while they were gazing into heaven as he went, behold, two men stood by them in white robes, 11 and said, “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 1:9-11).
That's presented as an eyewitness account, in observational language. That's what you and I would see, had we been there.
And it describes the return of Christ as a reversal of the Ascension. If we take the Bible seriously, we can't just discount the depiction of Jesus coming down from the sky.
vi) A critic might object that passages like Mt 24:30 and Rev 1:7 presume a flat-earth perspective. If the world is a globe, how could Jesus be seen all at once by everybody on earth? But if the earth was flat, then everyone would enjoy the same vantage-point in relation to the sky. Everyone would see the entire sky, facing the earth.
Yet even if, for the sake of argument, we grant that this imagery reflects a flat-earth cosmography, that might be an accommodation to how ancient people thought about the universe.
Mind you, I doubt there was any one way that ancient people viewed the universe. Many people probably operated with naive realism, but some people were more reflective and attentive difficulties with that viewpoint.
vii) However, it doesn't take much imagination to see how these descriptions are consistent with modern astronomy. If the sign of the Son of Man appeared in the sky for as little as one rotation period, everyone would be in a position to see it over the course of 24 hours.
Suppose the sign was like an approaching comet. Everyone would see it weeks in advance.
In the age of telecommunications, moreover, everyone can see the same thing, even if that's out of range of where they live–or the skies are overcast where they live.
viii) This, in turn, suggest a practical function for Jesus coming down from the sky. Suppose Jesus simply appeared on earth. How would anyone know that's the Second Coming of Christ? Outwardly, he looks like an ordinary human being. Moreover, there's a sense in which he only be seen at one place at a time.
Suppose, however, the "sign" of the Son of Man approaches earth from outer space. It isn't just ancient people with an interest in astrology who were impressed by portents and prodigies. Modern people with an interest in astronomy are impressed by portends and prodigies. Take speculation about an impact event that may extinguish life on earth, if a huge astroid strikes the earth. Or take recently speculation about whether we're receiving radio signals from an alien civilization.
Suppose the Shekinah initially appeared in outer space, visible from earth. At a distance, it might seem like a natural phenomenon. Yet astronomers are baffled, because it doesn't fit the profile of a comet, astroid, supernova, &c. As it comes closer, it doesn't resemble any natural astronomical phenomenon. And it's trajectory is naturally inexplicable. Of course, many people might initially interpret this as a flying saucer or fleet of flying saucers.
ix) Another factor might be additional events or supernatural phenomena, as an unmistakable precursor to the Parousia. The fulfillment of ancient oracles regarding the Antichrist, or things like that.