Suppose one considers a subset of his own beliefs: 1. God exists. 2. The Bible precludes theistic evolution. Now let’s further suppose that he holds (1) with a strength of .9 probability (that is, he is quite sure, if not 100% certain, that God exists) and holds (2) at .7 (that is, he is reasonably sure of its truth, though less so than ). Now this gentleman comes to believe, through arguments and evidence (whether good arguments and evidence are involved will be irrelevant in this case), this claim: 3. Evolution is true.
These truths are actually in tension; if (3) is true, then it is not the case that both (1) and (2) are true, provided a further claim: 4. The Bible is not mistaken with respect to (2). Now beliefs (1-4) form a contradictory set. If all four statements are correct, then whatever accounts for evolution is not theistic; but if nontheistic evolution is true, then God had nothing whatsoever to do with the origin of life (otherwise, it just would be a particular form of theistic evolution). In that case, arguably, God does not exist. So, the man in question should jettison (1), correct? Not at all.
Suppose the man holds (3) at .8, and he holds (4) at .9. In that case, he believes most strongly that God exists and that the Bible is not mistaken in its teachings concerning evolution and creation. But notice what the man holds to be the least plausible: the claim that the Bible precludes theistic evolution. In this particular case, he ought to give up his particular interpretation of the passage, and thus hold (1, 3, and 4). Or perhaps he, upon re-evaluation, comes to doubt (3) itself, so that evolution is the belief dropped.
My point is that even if evolution comes to be believed by the Christian, he need not jettison his belief in God. This is not merely pragmatic: there are very good reasons to hold that God exists (arguments for his existence, for example). It wouldn’t do to suggest that “evolution is true” is a defeater for the kalam cosmological argument, or moral arguments for God’s existence. Nor would evolution’s truth count against the strength with which the man held the other beliefs (that is, the ones that are compatible). Instead, other considerations ought to be brought to bear (such as the individual reasons for holding each of the other beliefs). We may discover, in fact, that we hold all of the other beliefs higher than we do evolution’s truth, so that evolution, while initially quite plausible, is nonetheless the belief discarded.
Too many Christians hold a “reverse confirmation bias,” where virtually any claim made by a skeptic counts as evidence against Christianity if those claims are even remotely plausible. It is actually irrational to hold Christianity to a standard that demands proof beyond all possibility of doubt. We must examine claims made by the skeptic to see if they really are incompatible with Christianity. If they are not incompatible, then we must ask ourselves what, if anything, we must give up if the claim is true? If it is incompatible (or incompatible with some other truth we hold within Christianity), we must ask ourselves about what reasons we have to hold the truths in tension, and then jettison the one we have the least reason to believe. I, for one, believe I have far more reason to believe that “God exists” than I do that “naturalistic evolution is true,” and thus, even if I find evolution highly probable (which I do not), I would not disbelieve that God exists. At worst, I would assume theistic evolution, and at best, I would simply discredit evolution, no matter how plausible it seemed, because all of my other beliefs held in tension were more plausible.
Now, one concern is epistemic circularity: suppose I believe “God exists,” for independent reasons, but I only believe “the Bible is not mistaken,” because I believe God exists. In that case, since “the Bible is not mistaken” involves another belief under consideration, should I not count that against the claim that “the Bible is not mistaken”? Not at all, especially since I have independent reasons to affirm “God exists.” Since I have good grounds for thinking God exists, and hence (via argument) good grounds to think the Bible is not mistaken, then so much the worse for either my interpretation of the Bible or for evolution.