What do you do about North Korea? is a military police state the opposite of religion (as DCP would want to say) or is it its own special kind of religion (as a typical atheist would say)? What counts as religion can also get complicated for believers. I don't know the lay of the land now, but back around my mission days, the born-again Christians I knew would say that religion is bad. Religion = "works of the law". If you're "religious", then you're trying to "save yourself" so to speak. Rather, you're supposed to have a personal relationship with Jesus. You're not technically accountable to any Church or institution, although (eh hum) finding a church that "teaches the Bible" is encouraged. And what about non-believers who are accused frequently of being religious, and in fact, being cult members of "progressiveness"? And so, getting that definition nailed down isn't easy.
If we take this simple dictionary.com definition:
Then we can discard the institutional controversy and simply say North Korea, woke progressives aren't religious; flat earthers aren't necessarily religious, and Christians are religious even if they aren't bound to a church. But as you can see by the simple definition, if suddenly "religion disappeared" then that implies nobody has a "belief in and worship of God" anymore. And that might not be what the original question is trying to get at. (?)the belief in and worship of a superhuman power or powers, especially a God or gods.
Although, if suddenly nobody believed in or worshiped God anymore, what would they do next? I think you can get a whiff of that by looking at the impact of social media. I'm not saying people who find alternative communities don't believe in God, but for many people whose belief in God is tied to their local religious community, for those who basically have ditched that and found their identity online in a context that isn't explicitly religious by the simple definition, then that gives us an idea.