After Christians worldwide go through a great deal of suffering, Jesus will return. Since Christians worldwide are currently going through a great deal of suffering, I see no reason he can’t return at any time. In the gospels, Jesus described his second coming this way.
But in those days, after that tribulation:
The sun will go into shadow.
The moon won’t give its reflection.
The stars will fall down from the sky.
The heavenly powers will be shaken.
Then they’ll see the Son of Man coming
in the clouds, with great power and glory.
Then he’ll send out his angels
They’ll gather his chosen people from the four winds,
from the edge of the world
to the edge of heaven. —Jesus,
Or, as Matthew recorded this teaching:
Immediately after the tribulation of that day:
The sun will go into shadow.
The moon won’t give its reflection.
The stars will fall down from the sky.
The heavenly powers will be shaken.
Then the Son of Man’s sign will appear in the sky.
Then every tribe on earth will give itself a swift kick.
They’ll see the Son of Man coming
in heavenly clouds, with power and all glory.
He’ll send out his angels with a mighty trumpet.
They’ll gather his chosen people from the four winds,
from one horizon to the other. —Jesus,
Or we can look at the less-poetic version of it in Luke.
Signs will be in the sun, moon, and stars. On earth the nations, gripped by difficult things, confused by the sound of the sea and surf, their people holding their breath in fear and expectation of what’s happening to their civilization—for the heavenly powers will be shaken—then will see the Son of Man coming in a cloud with great power and glory. Pay attention when these things begin to happen. Lift your heads up: Your redemption is near!—Jesus,
Jesus’s apocalypse.
Since we know stars don’t literally fall from the sky—for if they ever came close to Earth, we’d fall into them—it’s clear Jesus is speaking in metaphor. This is an apocalypse, a prophecy or vision which uses one image to represent another. Which means we have no idea how much of this passage to interpret literally or not.
The sun and moon could go dark—or something like the sun and moon going dark would take place. The stars falling from the sky could mean a meteor shower, which is what we tend to mean by “falling stars.” Or some other dramatic astronomical event. Dispensationalists, who interpret apocalypses as if they’re secret codes, often figure since “stars” meant angels in once verse, it automatically means it here too: These are angels falling to earth from the great war in heaven.
Suffice to say some event is gonna take place. Likely it’ll be something everyone on earth can see, just like people back then could all see the stars in the sky. (Not so much nowadays, thanks to the brightness of some cities’ lights.) It’ll be an event no one will miss—like the sun and moon going out. It’ll be huge. Jesus’s return will in no way be secret and hidden.
“The heavenly powers will be shaken” likewise can mean all sorts of things. Freaky weather? Chaos in space? The overthrow of the existing heavenly order? (That’d fit rather well with the Dispensationalists’ war-in-heaven theory.) The overthrow of certain authorities which think their power reaches almost to heaven? (That’d fit with the idea Jesus returns to overthrow all existing governments.) Suffice to say Jesus’s return is going to reorder the universe. God is coming to rule over his people in person. Nothing’s gonna be the same. Good.
The angels will gather Jesus’s people from the four winds, from as far as they can collect us. This’d be the rapture, when we take Jesus’s side in his invasion. We’re not gonna be left out of these events. Even if we’ve died; we’re all going to be resurrected. That’s what Jesus means when he said, in Luke, our redemption is near: Once we’ve been transformed, we’re absolutely, no question, in the Kingdom. No more “someday.” No more “maybe.” There’s nothing more to be done for our salvation: We’re going to live forever, for the new improved bodies we find ourselves in don’t die.
You notice, because this is an apocalypse, we’re limited in the number of definite things we can say about it. But even so, there are things we can draw from it. Great, hopeful things. That’s why we can follow Jesus’s command to pay attention. No, we’re not gonna miss it. But we want to be ready for it.