In a previous post, I emphasized the fact that on Plantinga's account of warranted belief, the degree of warrant a given belief enjoys depends on the degree of firmness with which one holds it. Thus, Plantinga's view entails the following two (roughly stated) theses:
I. Conditions of warrant are met + high degree of firmness = high degree of warrant.
II. Conditions of warrant are met + low degree of firmness = low degree of warrant.
Now Plantinga characterizes his account of warrant as externalist. But while thesis (I) is in accordance with his externalism (e.g., one can be warranted in firmly believing that Christianity is true if it meets all of Plantinga's proper functionalist conditions of warrant, even if one cannot know that (say) the faculty responsible for that belief is successfully aimed at truth), it appears that thesis (II) is not. For one can, at least sometimes, be aware by armchair reflection alone that one does not have a sufficiently firm belief that Christianity is true, and thereby infer that one's belief has little by of warrant. And if that's right, then Plantinga's theory of warrant isn't purely externalist.
One implication of this is that if Plantinga's theory of warrant is correct, then one can tell, at least under certain conditions, whether one's Christian faith is warranted. So, for example, if one is aware that one's belief in Christianity is of the fairly common weak and waivering sort ("I do believe; help thou mine unbelief"), then one can rule out that one's belief has enough warrant sufficient for knowledge.
(UPDATE: I have since read an old paper by Linda Zagzebski, in which she makes virtually the same point about asymmetry I bring up here. See her "Religious Knowledge and the Virtues of the Mind", in Zagzebski (ed.) Rational Faith: Catholic Responses to Reformed Epistemology (University of Notre Dame Press, 1993), pp. 199-225, esp. pp. 201-202.)
 There are of course very many characterizations of internalism and externalism in the literature, but discussion of such would distract from the point I'm making. It's sufficient for my purposes to speak in these rough and intuitive terms.
 Michael Sudduth has made a somewhat related point re: the partially internalist character of Plantinga's account in (e.g.) "The Internalist Character and Evidentialist Implications of Plantingian Defeaters", International Journal for Philosophy of Religion 45 (1999), pp. 167-187.
 I'm often astonished, though, at how frequently Christians underestimate how low their degree of belief in the truth of Christianity often really is. My guess is that they often think of belief in terms of an implausible mental-assent account (what Christian philosopher Dallas Willard memorably calls "barcode faith": the believer mentally assents to a set of propositions about their faith, and then God "scans" their minds and grants them salvation), when belief (especially the degreed notion of belief Plantinga has in mind in his account of warrant) plausibly has satisfaction-conditions that are much more stringent than that. The point I'm making in this post can thus be seen more clearly against the background of some of the most important recent literature on the nature of belief. I therefore refer the reader to these papers on belief by philosopher Eric Schwitzgebel (UC Riverside). If Schwitzgebel is anywhere near the truth about the nature of belief, then, again, it's implausible to think that many Christians believe with sufficient firmness to have warranted Christian belief, even if Plantinga's account of warrant is correct, and even if they meet all the other conditions of his account. The faith of a mustard seed may or may not be sufficient for salvation, but it's nowhere near sufficient for warrant.
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