Thought I had it too, but after starting to type it out realized there was a flaw in my logic.

So basically my understanding of this after some more reading:
People are thinking that the rapture happened, so one of Jesus' people are telling them it hasn't since there needs to be a powerful anarchist and/or Anti-Christ that comes about (the "lawless man"). The anarchist is gonna have powerful magic and will be preaching that he'll give the people a better life than god or whatever to get them off the path of god.
Okay cool, so the people (who are pre-sorted by god into the godly and condemned) will see this guy and some will be tempted by the lawless man, but won't necessarily do anything to deviate from.gods path. So god is gonna come along and do something* to make them actually deviate from the path of god (which is the delusion).

*the specific Greek phrase used in the original text is only used once here and so there isn't a good bead on what that delusion is/could be. The key thing appears to be that god will actively do something to make the damned be damned.