"...Consistency, my dear Mr. Brocklehurst; I advocate consistency in all things."
"Consistency, madam, is the first of Christian duties..."
--Jane Eyre, ch. IV
Everybody knows that they're supposed to laugh here. The joke is usually assessed in one of two ways: 1) this is a flagrantly obvious violation of Christian priorities (e.g.); 2) this indicates hypocrisy (e.g.). I became rather curious about "consistency" some time back, and while 2) is obviously the case--Bronte wallops us over the head pretty hard about the distance between Brocklehurst's theory and practice--I think that the novel's denunciation of Mr. Brocklehurst and Mrs. Reed is targeted a little more specifically than just general hypocrisy.
"Consistency," which conjures up dreary visions of gloomy, unsmiling believers, meant something a bit less glum than that: it referred to the believer's constancy in regulating all actions according to the Bible. As the Christian Penny Magazine put it, "A Christian may be said to be inconsistent, when one part of his conduct does not agree with another, or when any part of it is at variance with the Word of God." Manuals and advice books across all denominations exhorted readers to aim for the ideal goal of what Congregationalist John Angell James called "uniform piety." Thus, while it sounds bizarre to say that "[c]onsistency [...] is the first of Christian duties," it actually isn't--far from turfing out love in favor of some weird rules-oriented regimen, the call to consistency advocates fully incorporating Christian faith into every aspect of human existence. In other words, the reference to "consistency" per se is not the joke. Nor is it that Brocklehurst and Mrs. Reed are just hypocrites--beyond their own obvious failings, they are both obsessed with what Mrs. Reed calls the "system" at Mr. Brocklehurst's school, which certainly is an inflexible, rules-oriented regimen that has little in common with Biblical teachings. Their mutual misunderstanding of consistency is arguably the novel's first example of idolatrous behavior, insofar as they're elevating Mr. Brocklehurst's manmade "system" over divine truth. The true punchline may be the adult Jane herself, whose ability to regulate her own passions according to a conscience shaped by the laws of God qualifies her as truly "consistent" (pace Lady Eastlake) in a way that escapes Brocklehurst and Mrs. Reed alike.