I don't know if you believe in the 'Myers Briggs' stuff (I do to an extent) - but one of the most valuable tools with it (which isn't often mentioned) is to look out for any of your types switching from one to the other.
If this is happening it means you are under stress, and once you can spot it and do something about it, it's really powerful.
That's interesting. I've always tested as an ENTJ ("The Executive or Field Marshall). Lately I've been under a lot of stress as my startup has gotten zero traction and something needs to give soon, because I'm hungry (for food!). Anyways, I recently took the Myers-Briggs again and the result was INFP - which some websites refer to as "The Idealist" who are focused on searching for the value they can provide in life. Weird.
Myers Briggs is a descriptive, not prescriptive, categorization of personality types; belief doesn't enter into it. Like all categorizations, it's merely a useful description of people with a fixed precision of 16. If 16 is an accurate enough categorization for a particular need, then Myers Briggs is sufficient. And like all categorizations, it's just a single way of looking at things, it's not in any way authoritative. You can't dis-believe in a categorization system.
I think the GP was talking about belief in the usefulness (or reliability or validity) of Myers Briggs. Note that the MBTI doesn't just classify people into 16 categories; it classifies people into discontinuous binary categories along 4 axes, obviously resulting in a total of 16 categories. It's certainly useful to think about peoples' positions along those 4 axes, although maybe there are other axes more useful for describing personalities in some situations. However, I find the idea of absolute binary classification (i.e. you're either an introvert or an extrovert, there are no shades of gray) hard to swallow.
If this is happening it means you are under stress, and once you can spot it and do something about it, it's really powerful.