We're all familiar with Plantinga's account of warrant. As he summarizes it:
"Put in a nutshell, then, a belief has warrant for a person S only if that belief is produced by cognitive faculties functioning properly (subject to no dysfunction) in a cognitive environment that is appropriate for S's kind of cognitive faculties, according to a design plan that is successfully aimed at truth." (Warranted Christian Belief, p. 156)
However, it bears emphasizing that this isn't the whole story about his account. For of course warrant is not the same thing as knowledge for Plantinga; rather, warrant is "that quality or quantity, enough of which turns true belief into knowledge."
Relatedly, Plantinga's account ties degrees of warrant to degrees of firmness of belief:
"We must add, furthermore, that when a belief meets these conditions and does enjoy warrant, the degree of warrant it enjoys depends on the strength of the belief, the firmness with which S holds it." (Ibid.).
But of course there is a fairly wide spectrum of degrees of belief. Putting these points together, we see that on Plantinga's account, a given belief can meet all the conditions of his account of warrant, and yet have nowhere near the degree of warrant required to "turn true belief into knowledge".
Now here's the rub: even if it should turn out that Plantinga's account of warranted Christian belief is correct, and even if Christian belief typically satisfies Plantinga's conditions of warrant, such belief may yet have nowhere near enough warrant to constitute knowledge.
Billy Joe is reading 2 Corinthians 5:19 (with a humble and contrite heart, say, for good measure), where the author says that God was in Christ, reconciling the world unto himself. In this situation, Billy Joe forms the belief that
P. God was in Christ, reconciling the world unto himself.
Furthermore, P is produced in Billy Joe by properly functioning, (successfully) truth-aimed cognitive faculties, and in an epistemically congenial environment for forming beliefs of this sort. Unfortunately, though, Billy Joe's belief is of the "I do believe; help thou mine unbelief" sort, i.e., relatively weak, wavering, and fragile.
Thus, although Billy Joe's belief enjoys a smidgin' of warrant, it's nowhere near enough to constitute knowledge. And one worry is that most Christians believe with a degree of firmness that more closely approximates Billy Joe's than that of Plantinga's ideal believer.
(I should note that this point about Plantinga's account of warranted belief is not a new one; in fact, many Christian philosophers have made it. Examples include (the late) Phillip Quinn, James Sennett, Michael Sudduth, Keith DeRose, Andrew Chignell, James K. Beilby, Sandra Menssen and Thomas D. Sullivan.)