In my experience, most opponents and skeptics of theism reject theistic arguments on less than epistemically justifiable grounds. For example, premise one of the kalam cosmological argument proposes that “everything which begins to exist has a cause” (and concludes that since the universe began to exist, the universe has a cause). Some detractors of the argument will counter that since our only experience with cause and effect is within the spatio-temporal world, we cannot be certain that causation is possible outside the spatio-temporal world. While I think this is a fair point to consider, does it really undermine the premise, and hence the conclusion? It doesn’t seem to me that it does. While it is possible that the principle of cause and effect does not apply beyond the temporal framework of our universe, unless one can demonstrate that non-temporal causality is incoherent/impossible, the mere logically possibility that the principle of causality does not hold outside of the universe does not override the warrant we have for thinking all effects require an antecedent cause (and that contingent things require an external cause).
William Lane Craig has offered a good set of criteria for what constitutes a good deductive argument, as well as what kind of epistemic justification is required before one can legitimately reject the conclusion of such an argument:
Now keep in mind what I mean by a “good argument.” I mean an argument which (i) is logically valid; (ii) has true premisses; and (iii) has premisses which are more plausible than their negations. In order to show that an argument is no good, it is not enough for the sceptic to show that it’s possible that a premiss is false. Possibilities come cheap. I’m puzzled that so many laymen seem to think that merely stating another possibility is sufficient to defeat a premiss. This is mistaken, for the premisses of an argument need be neither necessary nor certain in order for that argument to be a good one. The detractor of the argument needs to show either that the premiss in question is false or that its negation is just as plausibly true as the premiss itself.
Just because it is logically possible for a premise to be false does not mean one is justified in rejecting it. So it is with premise one of the kalam cosmological argument. Given the preponderance of the evidence, there is neither reason to believe the premise is false, nor that its negation is just as plausible or more plausible than the premise itself. As such, one would be intellectually dishonest to dismiss the kalam argument on the basis that one of its premises cannot be known with certainty. After all, such is the case with virtually all premises.