Writing partner and I were recently working on a piece that involved a character escaping brainwashing like that and we had a convo about this exact thing. Because I thought the character need more than one thing to break the brainwashing, and partner, who has actual experience escaping a cult she grew up in, said something very interesting. 'Not if there were cracks.'
She went on to explain how for her, there had always been little things about the cult that bothered her, but she excused them, pushed them aside, papered them over in her own mind. And then one thing happened. One very specific thing that went directly contrary to all the things the cult taught her.
And that one thing put enough pressure on her belief that all those cracks... exploded and she lost faith in the cult practically overnight. And the thing is, from my outsider perspective, it was like one day I talked with her and she was true believer, and the next time religion came up she was an ex- and talking about how she'd just recognized this thing about how harmful her former religion was.
I knew another man, briefly, who grew up evangelical protestant, spent more than half his life living and proselytizing as a true believer, then one day just... stopped. He said that he had always had doubts, never really believed, but he pushed all those doubts and disbeliefs down and acted all the more fervent to prove to himself that he was a good Christian. Until one day he realized what he was doing and... was done.
Now, with writing, we truly hope an author is good enough to convey this kind of internal conflict, but when someone spends half their life suppressing these kinds of things, it can be very hard to see even from the 'inside', because hiding it from themselves is the whole point. And when aren't talking about a PoV character or are in a real world situation...
Not long ago, i would have agreed with you. Now I can say that actually, sometimes I can be 'just one thing' -- or at least look that way because all the little things that came before are so small they're invisible.