Recently, I was reading Gordon Lewis' summary and critique of Gordon Clark's apologetic in Testing Christianity's Truth Claims. Clark said Lewis' representation of his position here was done "better than any other critic" (Clark and His Critics, 2009, pg. 394). Clark provides his own response to Lewis in that book. I just want to highlight a few points related to Clark's apologetic I think should be made clearer.
To begin his final section of his chapter on Clark - his "Evaluation" - Lewis writes: "Suppose for the moment that consistency is the sole test of truth-claims, as Clark asserts." (Testing Christianity's Truth Claims, 1976, pg. 119). Clark points out this isn't true. In one of his earliest books (originally published in the late 1940s), for example, Clark wrote: "While consistency is one of the basic reasons for adopting a world-view, from a more proximate standpoint the world-view must function as a practical postulate" (A Christian Philosophy of Education, 1988, pg. 42). For some reason, this tends to be an overlooked point.
For Christians, apologetics serves a certain function: "to defend the truth of Christianity against the attacks of its enemies" (link). Attacks can come in different varieties and relate to different fields, but basically, they all question that Christianity is true or knowable. If an apologetic has nothing to say about these different fields and has no relevance to whether Christianity is true or knowable, it isn't practical.
The other point I want to make also relates to the function of apologetics. Lewis seems to make the mistake of equating Clark's "test[s] for truth" with how a Christian knows the truth. He mistakes Clark's apologetic for Clark's epistemology. Lewis writes:
Admittedly, Christianity's truth-claims cannot be proved by inductive evidence. But Clark chooses to believe their truth because Christianity, of all the systems men have known, is alone consistent. Notice what is necessary for Clark to establish that thesis. He must show the inconsistency of every other system in history and on the contemporary scene... On what grounds does Clark know that there could not be two or more consistent systems? He assumes that only one system could possibly be consistent. (Testing Christianity's Truth Claims, 1976, pg. 119, 120)
Note that the following isn't true: "Clark chooses to believe their truth because Christianity... is alone consistent." Rather, Clark chooses to argue their truth because Christianity is alone consistent. This is the difference between apologetics and epistemology. Apologetics consists in making arguments. This is not always so in epistemology - axioms are not known because they are the conclusion of some argument, they are known because they are self-authenticating.
Clark has to argue for Christianity via logical consistency and practicality because, at the risk of stating the obvious, he can't know Christianity for those to whom he is engaging in apologetics. He could just say divine revelation is self-authenticating and leave it at that, but it is more persuasive (which is another function of apologetics) to additionally point out, when applicable, that an opponent's system 1) can't be self-authenticating if his system inconsistent or impractical, or 2) that his system is less coherent than Christianity is, in the case of something like Judaism.
Lewis seems to think that Clark's tests for truth are the basis on which Clark claims to know Christianity. That is really the only explanation for why Lewis would think Clark would have to sift through infinitely many systems before knowing that just one, Christianity, is consistent. But that interpretation goes completely against what Lewis himself stated Clark believed regarding the nature and knowability of axioms earlier in his summary.
While "test[s] for truth" can serve as confirmatory evidences of Christianity, they shouldn't function as the ground of knowledge; divine revelation does. Elsewhere, I have called tests for truth necessary conditions for knowledge and the postulate[s] by which one claims to know anything the sufficient condition[s] for knowledge (link). The former are the means by which we make arguments for (i.e. apologetics) the latter (i.e. epistemology).
Again, apologetics should include an explanation of the epistemology of the system one is defending. Any good defense of a system of knowledge should explain what that system says about how we can know anything. But that explanation and defense should not be confused for that actual process of knowing. In fact, apologetics is only possible insofar as we know the system we are defending is true in the first place (see here).
As for where Clark stated these points - that axioms are known because they are self-authenticating, not the conclusion of an apologetic, and that consistency isn't the [sole] means by which we know Christianity is true - a few quotes should suffice:
This disjunct faces two replies. First, it assumes that a first principle cannot be self-authenticating. Yet every first principle must be. The first principle of Logical Positivism is that a sentence has no meaning unless it can be verified (in principle at least) by sensory experience. Yet no sensory experience can ever verify this principle. Anyone who wishes to adopt it must regard it as self-authenticating. So it is with all first principles. (Christian Philosophy, 2004, pg. 47)
Undoubtedly I hold that truth is a consistent system of propositions. Most people would be willing to admit that two truths cannot be contradictories; and I would like to add that the complex of all truths cannot be a mere aggregate of unrelated assertions. Since God is rational, I do not see how any item of his knowledge can be unrelated to the rest. Weaver makes no comment on this fundamental characteristic of divine truth.
Rather, he questions whether this characteristic is of practical value, and whether it must be supplemented in some way. It is most strange that Weaver here says, “I must agree with Carnell,” as if he had convicted me of disagreeing with Carnell by providing no supplementation whatever. Now, I may disagree with the last named gentleman on many points, but since it is abundantly clear that I “supplement” consistency by an appeal to the Scripture for the determination of particular truths, it is most strange that Weaver ignores my supplementation. (Clark and His Critics, 2009, pg. 290)