[PatrickHenry]

7. Mysticism is one of the great forces of the world's history. For religion is nearly the most important thing in the world, and religion never remains for long altogether untouched by mysticism.
— JOHN MCTAGGART ELLIS MCTAGGART, "Mysticism," Philosophical Studies

Another subtle one. To restate it:

1. Mysticism is one of the great forces of the world's history, because:
2. Religion is nearly the most important thing, and
3. religion never remains for long altogether untouched by mysticism.

So, McTaggert says that because mysticism is usually (but not always) associated with religion, and because we know (it's presumably a given) that religion is important, therefore mysticism too is important. Hmmmmm. The assumption here is that a frequent (but not constant) component of religion is important because religion itself is important. A characteristic of the whole (importance) is being attributed to a portion of the whole.

This is the fallacy of division, arguing fallaciously that what is true of a whole must also be true of its parts.

[general_re]

You know, I don't actually have the (official) answers here, so I've been meditating on some of these myself. Some are immediately obvious, and some are rather more subtle. I like your reasoning here, and I agree with the case you make, even though I arrived at a rather different assessment, which I will present without the Socratic interrogation this time ;)

The key, to my mind, is when McTaggart essentially defines religion and mysticism as equivalent. It goes like this:

P: Mysticism is religion, and;
Q: Religion is important, therefore;
R: Mysticism is important.

The trick is that "mysticism is important" and "religion is important" are logically equivalent once you define religion as equivalent to mysticism - If "religion=mysticism", then one proposition is what you might call a suitable paraphrase of the other. In a more bare-bones form, the argument is:

P: X
Q: X=Y
R: Therefore, Y.

But if X and Y are really equivalent, then X and Y have, effectively, the same propositional content - they are, in a real sense, the same identical proposition in both cases. And if X and Y are the same proposition, then saying "X=Y" is really the same as saying "X=X", and the argument is revealed as:

P: X
Q: X=X
R: Therefore, X.

So what you're doing when you define religion as important, then say that religion is equivalent to mysticism, and derive from that the conclusion that mysticism is important, your conclusion is logically equivalent to your first proposition - the conclusion is simply a clever restatement of one of the premises. And that's begging the question - you cannot assume the truth of the conclusion in your premises.

This is one of my favorite examples - we tend to be presented with trivial examples of petitio, where the circle is blatant and obvious. But most people are smart enough to avoid blatantly coming out and saying "X=Y=X", and this is a good example of how damned subtle the fallacy of begging the question can be.