Wow, i never thought of Gellert as ENTP it would explain somewhat why Albus may have 'liked' him because ENTP/INFJ's are natural partners... but he was my favourite character in DH.@EmpireConquered: All right, so, here's my reasoning for typing Grindelwald as being either ENTP or INTJ, in full. I'm going to be using quite a few passages from the story, and from other canon sources.
Now, before I begin to write about why I think he's either ENTP, or INTJ, let me explain why he decidedly isn't ENTJ. I do not think he's a Te-dominant; his style of persuasion and motivation was very feelings-oriented, with a focus on the ideal, the possible, the ideal world of tomorrow. When Te is being used to persuade, it convinces the other person's mind to realize the practical sense of the person's statement; that's why saying that Te-doms don't manipulate others is misleading. They do, but it's not being used to affect your emotions, but appeal to your sense of rationality, that the only logical way forward is their way, often by engineering situations where this is the case. You can see quite a bit of this style of persuasion in Voldemort: whenever he wanted to achieve a desired result, he organized situations where people had no choice but to do exactly as he wanted them to, or would show them a part of the truth, allowing them to draw the logical conclusion he wanted them to, and act as he believed a fully rational agent would in that situation.
Take the diary-Riddle, for example; he presented an amicable, trustworthy facade in order to gain Harry's trust, but when it came time to manipulate Harry into doing what he wanted him to, he did not directly appeal to his emotional side, but rather, showed him a memory of Hagrid's capture, which led Harry to draw the conclusion that Hagrid was the one who released the Basilisk. There was no Fe-style emotional persuasion going on, but Te-style "well, here are the inarguable facts. Therefore, this is the correct process to take if you want x result." Voldemort understood goals and processes, not emotions; he manipulated by engineering situations where the only process toward a desired outcome was his way, not any other.
Grindelwald, on the other hand, was very different. He preferred to bring people to seeing things his way by feeding their emotions, by taking them on wild trips of possibility, and by convincing them that they were ultimately doing the right thing, as opposed to the logical thing (to Grindelwald, the logic of wizards ruling over Muggles was self-evident). Take a look at this passage:
Here, you can see the still-lingering traces of Grindelwald's style of persuasion in Dumbledore's way of ironically disparaging the dreams he and Grindelwald once shared. These were the very thoughts going through Dumbledore's head as he spent that summer with Grindelwald, dreaming up a future society of wizard domination over Muggles. Grindelwald, unlike Voldemort, understood emotions, and understood them well enough to use them in persuading others to get behind the ideas he was coming up with. I see intuition, but the logic and the planning seemed secondary to the vision itself, so I don't think ENTJ is very likely, given the severe differences between Grindelwald and Voldemort. On top of that, Grindelwald's strength was as a primarily visionary leader, not as a great planner and/or organizer of people. I've explained where his weakness in this area showed itself most of all, further down this post.
So, that leaves ENTP and INTJ as possibilities (INTP is unlikely, as he seemed to have too great a grasp on Fe for it to be his inferior function). Arguments could be made for both sides, so I'm going to outline my own, right here.
The similarities that both types share is dominant intution, followed by Thinking, Feeling, and Sensing. The difference I've noticed is that INTJs see the end first, and then see the steps needed in order to make that future a reality, while ENTPs see multiple paths all leading to different possible results, choosing the most desirable end based on what makes the most logical sense to them. The question here is, was Grindelwald's vision an intuitive conclusion (Ni) being supported by his Te? Or, was it great fascination with an existing idea (Ne), which was exacerbated by finally having someone his age brilliant enough to understand and even help refine his vision to a point of logical consistency (I do think that, the more time an ENTP is willing to spend on developing a theory, the more he or she is going to want to put it into practice)?
I think it's the latter, for several reasons. Number one, upon reaching Godric's Hollow, I don't think Grindelwald had already begun planning his quest for world domination; the story makes it clear that he was already fascinated by the Deathly Hallows upon meeting Dumbledore, and it's highly probable that the idea of wizard dominance over Muggles had already implanted itself in his mind. I do think he had an idea of the future world he wanted to build, and he may have even been trying to get a headstart on making his grand vision a reality, but as of meeting Albus Dumbledore, no definite, concrete plot had been established. Plus, if a plan had been established, he spent far too much time deviating from its implementation, and far too much time refining his theories with Dumbledore, which suggests that, at the very least, he was an Extraverted Perciever of some kind, endlessly gathering information from the outside world. In fact, I would have said ENFP was also a possibility, except for the fact that Grindelwald's value system was mainly utilitarian, with some in-group loyalty thrown in there. ENFPs have more personal morals, and more personal convictions behind the things they do; his justifications would likely be more complicated than simply "For The Greater Good", while most ENTPs can relate to that statement, to some degree. I would expect an INTJ with grand ambitions on that scale to be more concerned with getting Dumbledore to help him with the nitty-gritty of the plan, perhaps making preparations to leave on a quest for one of the Hallows, not with refining the vision endlessly over a two-month period, and being primarily concerned with filling his head with dreams of becoming the harbinger of a new era. Dumbledore's letter (which I'll repost for convenience)
was about refining the idea to a point where it was more ethical, and where only the necessary force would be used to achieve his vision. From this snapshot, we can infer that this was the usual conversation between Albus and Gellert; imagining the future, Gellert throwing ideas out on what he'd want it to look like, and Dumbledore putting his own spin on things, while providing the necessary reduction to the correct path forward. This could also be a sign that he's an INTJ, because Grindelwald and Dumbledore's vision of the future world lined up quite neatly with one another, and the letter seems to be a debate between weighing the importance of efficient process (Te) and human suffering (Fe). However, I would also expect to see a clash between Grindelwald's Fi, and Dumbledore's Fe, which didn't seem to be the case; Dumbledore and Grindelwald both seemed to agree that personal feelings were subservient to the greater good, it was more an issue of whether the soundness of the vision had greater priority over the human costs that would inevitably arise.
Now, on to the second point. Grindelwald, for all his brilliance, had the deadly flaw of impulsivity, which was partly what led to his downfall in the end. Contrast Grindelwald's school career with Voldemort's; Voldemort was very cold, calculating, and made it a point never to get caught terrorizing (or, in Myrtle's case, killing) the other students. Grindelwald, on the other hand, was expelled for performing "twisted experiments" on his fellow schoolmates, though for what reason, we will never know. Based on what I understand of his personality, Grindelwald was probably experimenting on his schoolmates to acquire knowledge that would be difficult to learn in another manner. It was put in there primarily as a hint to the extremes that he was already willing to go to in order to support his ends. This is a sharp contrast between the cold, diabolical organizer (Voldemort, Te-dom), and the boy whose curiosity outstripped his compassion (Grindelwald, Ne-dom). Grindelwald acted in response to new information, even when inaction would have been wiser. Voldemort disregarded information he didn't see as relevant to his plans.
That's Aberforth narrating the events that sparked the duel which killed Ariana (remember this quote, because I'll be coming back to it later). Based on that, it sounds like Grindelwald was prone to allowing his whims in the present moment override his better judgment; this isn't to say that INTJs don't do this, but it's more common in ENTPs. An ENTP lives in a world of improvisation; we are constantly absorbing and reacting to new stimuli, and because we are so comfortable just winging it, we also have a tendency to allow ourselves to make choices that we could have easily avoided if we'd stopped to think things through quite a bit more. We're deliberate in our proaction, but our reaction is what often screws us, though it's also one of our best assets.
So, based on those two anecdotes, it's clear that Grindelwald had a tendency not to consider his decisions before making them; unplanned, and not methodical at all. This is evidence against him being an INTJ, and evidence supporting ENTP to a greater degree.