Regardless of how sure you are, do not call someone the Antichrist, the Whore of Babylon, or a Rider of the Apocalypse. It never helps.
But what if you’re really really sure? Isn’t it your job to tell the world, rather like if you knew that your neighbor was a murderer?
Not really. It’s your job to get a glass of water and go to bed and think about it some more when you’ve got some rest. Setting aside the general weakness of dispensational theory and other eschatologies that include an Antichrist (he is, after all, never mentioned under that name in the Bible), enough people have been wrong about this that you’ll probably end up as just another person who looks like a bigoted idiot in a couple of years. After all, it wasn’t Gorbachev, and he even had a crazy birthmark.
Of course antichrist-naming is a fringe activity generally restricted to fundamentalist conspiracy theorists and to people who don’t really mean it in earnest but like to use it as a slur to rally the like-minded behind. However, the Whore of Babylon is an oddly different case.
Even otherwise-intelligent thinkers seem to believe that they can identify the Whore of Babylon. Thus, we have the hook-line-and-sinker Reformed belief that it’s the Roman Catholic Church. If such respected theologians as Martin Luther, John Calvin, John Knox, and Jack Chick believed it, it must be a respectable belief. Except no, not at all. It’s not a respectable belief because it’s a useless belief. It’s nothing but a stick to beat institutions you don’t like with. After all, if an institution has actually done enough evil things to reasonably qualify as the Whore of Babylon, you could simply criticize it on that basis. All the name-calling adds is an excuse to write off the institution entirely. There’s no need to objectively consider whether its actions were really that bad if it’s the Whore of Babylon: Of course it’s that bad, and even if it isn’t, it will be eventually. It also negates the possibility of said institution changing or improving. Finally, it provides an excuse to never work with that institution or anyone who is a part of it, reinforcing the Reformed tendency to circle the wagons against theological opposition by denouncing it as heretical.
But this is a law of discourse, after all, not a law of personal belief. The best reason not to call someone or something by one of these names is simply that it makes you look like an ass. It gives anyone who likes whatever you’re criticizing, as well as anyone on the fence, a profoundly good reason to never listen to another word you say.