A lesson from the yeshiva bochur's playbook:
1) Torah is right and true, and that fact is patently obvious to everyone who bothers to think about it.
2) So, then, how could it be that people don't follow the torah?
3) Because they simply don't care enough about doing what's right.
4) Obviously, no one wants to think of themselves as someone who doesn't care about right and wrong, so in order to let themselves sleep at night, they rationalize and say that really they believe that the torah is not at all true.
5) But of course, we all know that the torah is totally true. From here we see the amazing power of the mind to rationalize and make us change our beliefs when the heart wants to do something that it shouldn't (a.k.a. following the yetzer hara).
This is a basic fact of the world that most any yeshiva guy would be able to tell you. "It's pashut," they would tell you. Simple. And just one of the many unflattering ways in which non-religious people are portrayed in the chareidi world - unprincipled, impulsive hedonists who do whatever they want and then come up with a justification after the fact to rationalize their behavior.
Putting aside the fundamentally flawed premise that the argument rests on, I would actually agree with part of that assertion. I tend to agree that people find ingenious ways to rationalize their behavior all the time. Cognitive dissonance is probably far more prevalent in our lives than we care to acknowledge. We don't ever want to admit that something we are doing may be wrong. But despite that concession, I don't think it's fair or accurate to look at the world the way the chareidi world does.
And specifically in this regard, I think there's another approach which might actually better explain why people stop believing certain things about Judaism when they stop following halacha: It's simply that for many people, the main reason they were believing those ideas in the first place was not out of a conviction of their truth, but rather because they needed to believe in them in order to justify how they lived their lives! And now that they aren't living that lifestyle anymore, they consequently have no need to believe the ideas anymore either!
Think about it: What could possibly justify putting ourselves through the burdens and nuisances of frum life? For most people, the only thing that could really make it worth staying committed to such a demanding responsibility would be if they believe that it really matters in some larger sense. But if they stopped having that massive yoke weighing down on them, they wouldn't have to believe that all of that stuff mattered!
Imagine a person who is trapped in an unhappy relationship, with no chance of ever escaping it. Because they're stuck in it, their mind comes up with all sorts of reasons why this relationship is actually a good thing - how it provides so many benefits, why no one else can see how wonderful it is, how it's the fulfillment of all their deepest wishes, etc. They need those explanations, because without them, the reality of their life is simply too distressing to acknowledge. But if somehow, through some unexpected yet fortuitous turn of events, they managed to escape their prison, do you think they'd retain those views? Would they actually look back on that relationship with any fondness?
This perspective is actually the exact opposite of the frum explanation. They see the situation as people starting with a core belief, and from that conviction their torah observant lifestyle arises - The actions derive from the belief. The way I'm seeing it now is people being straddled with a challenging and difficult lifestyle and needing some rationalization for why they have such a demanding life -The belief is born from the actions. So, according to the frum world, when people's beliefs change in response to the lifestyle changing, it's a result of dishonest rationalizing. The way I see it, when the beliefs change to match the actions, it's actually a more honest expression of who they truly are than when they imposed those beliefs on themselves. In fact, now that I think about it, it was back when they were believing all those ideas, that they were rationalizing more than ever!
Ironic, no? Chareidim look at the rest of the world and see endless rationalizing, yet it may well be that for so many chareidim, it's their ability to rationalize that lets them get by without having to face the inconsistencies and inanities of their lives!
You know, I think they may have been a bit mistaken about something they taught us back in yeshiva: It wasn't Torah that sustained klal yisrael through the ages. It was our ability to rationalize the absurdities of our existence.