God might have made us so that when we consider evidence for the non-existence of God or the unreliability of the Scriptures or the illusory nature of religious experience, the strength of our theistic belief would actually increase. Maybe all such evidence is in the end deeply misleading and God does not want us to err in matters of ultimate importance. So a student, call her “Faith", takes a philosophy of religion class from a brilliant atheist who presents convincing versions of arguments for all the above theses. She cannot see a thing wrong with any of them. But in accordance with her design-plan, the strength of Faith’s conviction in the central tenets of Christianity is thereby strengthened, not weakened. Indeed, perhaps with enough apparently sound arguments for the falsity of Christianity her belief will become maximally warranted!
Now Plantinga can, of course, say that her design plan is not like this. There are potential defeaters for God’s existence and the claims of Christianity, and we are not made to believe more strongly when we confront them. That is probably true (although given Plantinga’s assumptions about the damage that the Fall has done to our faculties where our belief in God is concerned, I am not sure how he can be confident in reading off the design-plan from our actual cognitive function). But, even if that’s right, the counter-example still remains in place. For Faith’s belief is produced by a successfully truth-aimed, properly functioning cognitive faculty operating in an appropriate environment and believed with maximal firmness. Plantinga’s epistemological theory entails that beliefs with these properties are maximally warranted – hence, they are as warranted as one’s belief in one’s own existence.
Senor, Thomas. “A Critical Review of Alvin Plantinga’s Warranted Christian Belief”, International Philosophical Quarterly 42:3, Issue 167 (September 2002), 395-396.