This is an excerpt of mine from an e-mail correspondence with an INTP friend who is kind of agonizing over the INTP vs. INTJ dichotomy and annoyed that IPs lead with a judging function (and IJs, with a perceiving one.)
I argued that MBTI's J vs. P is simply not the same thing as Jung's terms for perceiving vs. judging functions, but is its own unique fourth attribute.
Anyway here are my thoughts on J and P, if anyone has any response:
His inflexibility on Christianity is not characteristically INTJ; INTPs do this constantly on issues that they've spent so much time thinking about that they've mentally classified the issue as "solved." An INTP takes a long time to get to this conclusion with most things, much longer than an INTJ--they insist on seriously considering every bit of data they can find during the decision process--this is another difference from INTJ, who is much more willing to apply his theories and act on his conclusions with little to no experience and relatively little data. To spend too long in considering all the angles and thus fail to act would be inefficient, and that's where MBTI's J function comes in.
The kid is simply absolutely convinced that he's spent enough time on that issue and therefore found the correct answer, and now he's gone into INTP-certainty mode about it where he's very unwilling to listen to any new evidence or information. INTJs are not really inherently more rigid than INTPs; they're just harder to set into motion and they value efficiency so much that they often come to a fast decision in lieu of considering every possible angle (sounds like you are exactly right that Chris Anderson is INTP.) Only when new evidence arises indicating the previous conclusion was likely erroneous will the INTJ continue spending serious time deliberating new possibilities--but he reaches this point more easily than an INTP who's already "concluded" that issue in his mind.
The difference is pre-decision and post-decision preference. That's a good way of describing the MBTI J/P dichotomy--Ps are more comfortable pre-decision (that's why they take so long to make a determination in the first place), but become rigid post-decision because this is no longer comfortable territory. They're just really uncomfortable making definite, final decisions on anything important (trivial decisions aside) without being ABSOLUTELY SURE it was the right decision, so this fear of being boxed in combines poorly with the unfortunate fact that decisions must be made to exist practically. Ps may be so afraid of making wrong definite determinations that they convince themselves that the "definite decisions" they've made MUST be correct--the alternative is uncomfortable enough that they will sometimes deny new post-decision information in order to avoid accepting having made the feared "final judgment" incorrectly.
Js are the opposite--most comfortable post-decision, resulting sometimes in hasty judgments. I'm sure I don't need to elaborate further here.