Formally, a hypothesis rules out certain observations as impossible. Instances where we make such observations are of course evidence against the hypothesis. But instances where we look for such observations (or are passively exposed to the possibility of finding them) and do not in fact make such observations, are evidence in favor of the hypothesis.
Faith makes its own rules, it's not a set of epistemological principles, but rather is essentially opportunistic: it's whatever arguments make you feel good at the time.
At various key points in the Opinion, Creationism is charged with being untestable, dogmatic (and thus non-tentative), and unfalsifiable. All three charges are of dubious merit. For instance, to make the interlinked claims that Creationism is neither falsifiable nor testable is to assert that Creationism makes no empirical assertions whatever. This is surely false. Creationists make a wide range of testable assertions about empirical matters of fact.
Well, this raises an interesting point which I shall try to make relevant.
We often distinguish between science-as-a-collection-of-facts and science-as-a-method-of-inquiry. In the same way we can distinguish between creationism-as-a-collection-of-falsehoods and creationism-as-a-method-of-being-wrong.
It is true that creationism-as-a-collection-of-falsehoods contains many propositions that can readily be falsified by application of the scientific method. By application of the scientific method. But creationism is not merely a collection of falsehoods, but a method of being wrong, and this is what makes it unfalsifiable.
For the creationist is always allowed to add an auxiliary hypothesis to reconcile his core beliefs with the evidence. For example:
* Dating methods reveal the rocks to be too old for creationists' tastes. Let's add a hypothesis: somehow the miraculous events of the flood caused "accelerated radioactive decay".
* Many stars are measurably too far away for their light to have reached us by now if the universe was young. Let's add a hypothesis: God miracled the light into existence in transit. Or, alternatively, light sprinted most of the way and has slowed down to c just as we started measuring it. For no apparent reason.
* It would be impracticable to fit all those animals on the Ark. Let's add a hypothesis: the measurements given in the Bible are of the external dimensions of the Ark, and God miraculously made it bigger on the inside than the outside. (I didn't make that one up.)
And so on. Now in fact any hypothesis can be defended in this way. If someone wanted to defend the hypothesis that there is a giraffe in my back yard, in the face of the evidence that I can't see a giraffe in my back yard, they can always add a hypothesis: "But what if this giraffe is invisible! What if God is making the giraffe invisible by magic!" But no-one is motivated to defend the giraffe hypothesis in this way, whereas people are motivated to defend the core notions of YEC in this way. This is why Overton was right to say that creationism is dogmatic and non-tentative: the core notions are fixed, and the work of creationism is to make them unassailable by the addition of auxiliary hypotheses. And this, of course, makes creationism unfalsifiable. For false statements combined with this method of adding auxiliary hypotheses ad hoc, is indeed unfalsifiable.
And this is the problem with faith generally. As my example of the giraffe shows, it is possible to believe literally anything (except perhaps a flat contradiction in terms) so long as you are sufficiently attached to the idea that you will add any number of auxiliary hypotheses rather than abandon it.
This applies to matters of fact, as creationists have so ably demonstrated (there's six words you've never seen together before!) It applies to the explaining-away of awkward texts: you can make them mean practically whatever you mean by proposing novel interpretations of the words, ad hoc, to change the meaning of the passage. (See here for an example of John Calvin confronting and disarming the very unCalvinist Biblical text "God will have all men to be saved".) It applies to moral questions. If some pesky atheist starts complaining about (say) the Holocaust as an example of the Problem of Evil, then the theist can say, ad hoc: "Perhaps that was really a good thing, for ... uh, reasons? Anyway, who are we to say what's good or bad?" Of course, no-one would say that in my defense if I'd been an accessory to the murder of six million people, because no-one is motivated to do so any more than they are motivated to defend the giraffe in my back yard. But when they need to defend a matter of faith, they always can.
This does set up a conflict between the entire method of science and scholarship on the one hand, and faith on the other. Someone bound by ordinary reasonable methods of inquiry would reject outright many religious propositions rather than build up an edifice of ad hoc hypotheses to defend them. Faith, on the other hand, requires certain propositions to be sacrosanct, and requires that the believer should engage in whatever intellectual maneuvers are needed to defend them.