Michael Martin says that “any appeal to obvious or self-evident evidence must be regarded with suspicion, for many things that have been claimed to be self-evidently true…have turned out not to be true at all”. (p. 83)
What Martin is saying here is that the so called truth of Christianity is based on something Christians claim to be true in a self-evident way and that people should be skeptical of this sort of thinking. The self-evident truth that Christianity is built on and that Martin refers to is the ultimate authority of God and His revelation. Conceptually, this idea is called foundationalism. What critics of Christianity like Martin don’t acknowledge is that all people adhere to foundationalism. That is, all people hold some beliefs as properly basic, justified true beliefs. This includes non-theists. Atheists will object to this of course, because they feel like they exemplify non-belief. Yet, even atheists maintain foundational beliefs such as naturalism being all there is or that science is the ultimate authority. So, the question is why people choose certain foundational beliefs over others. Obviously, the naturalist is going to say that science is palpable and can be comprehended by the senses whereas metaphysical beliefs, like God, cannot. This is not totally true. Metaphysical things like God can be experienced by the senses and by rationality. They just require a different set of noetic tools than naturalism. Aquinas calls this the lumen intellectus agentis in his Commentary on the Sentences of Peter Lombard (I Sententiarum - Question 1, Article 3). Furthermore, we know that the naturalist worldview cannot account for things that we know to be true such as moral sense or the mind/body problem. To turn to the non-theist worldview is to ignore the shortcomings of that worldview.
In the end, Martin’s critique isn’t valid. To criticize Christian foundationalism is to assume another set of foundational truths which are, at most, equal to Christian foundational beliefs and, at worst, less true than Christian beliefs.