Richard writes: “Suppose someone comes to faith by way of the KCA and the evidence for the Resurrection. What if we find out later that the KCA is a false argument and the evidence we had for the Resurrection which inspired this person's faith was bunk? I don't think it's enough to say, "Well, it was ultimately the Holy Spirit that guided them and not the arguments." -- true. But what about the vehicle used? There is something to be said about the potential lead someone to Christ under false pretenses.”
Randy: This is a good question! The question will be, I think, whether or not he was justified in coming to Christ. And that justification is plausibly there even if there were no arguments. But let's set that aside. The hypothetical believer could be justified in taking the kalam to be a sound argument, even if it ultimately turned out to be false (after all, people have justified false beliefs all the time). And if, in concert with other beliefs (that will be entailments for the kalam and Resurrection), that leads him to Christianity, he would then have been justified. The real interesting question is whether such a believer would retain justification in light of the falsehood of the kalam (or whichever of the arguments worked for him). If his only source of justification were those arguments, and those arguments were to be unsound, then even if the conclusions were true, he wouldn't be justified.
So, here's how it breaks down: initial conversion of the new believer: justified; new believer comes to believe the arguments are unsound; new believer has no other source or reason to think he is justified--then at that point the new believer would be unjustified in holding to faith. The believer who uses the arguments is only acting irresponsibly if he thinks the argument's premises are not sound or are dubious, but uses them anyway (that would be the false pretenses). As the kalam stands, it seems one is justified in thinking it is a sound argument at this point, so that even if it turned out to be false, I wouldn't thereby be deceiving someone. Only in the case that I believe it is unsound and use it anyway would that apply.
But I think this question highlights a very important point about justification and the believer. I think a believer is justified in taking the truths of the Gospel to be true even in cases where no arguments or evidences establish the conclusion that “God exists” just in the case that it is actually true God exists. Such a project was the task of Alvin Plantinga’s Warranted Christian Belief. This means that any complaints about a believer being unjustified are going to be about the de facto question—that is, the question as to whether or not God exists in fact. This means the dismantling of all theistic arguments will not be sufficient to show that a believer is unjustified in holding his faith: what will make him unjustified, or unwarranted, really (there is a definite difference in Plantinga’s account), is if one can establish that God does not exist. In the absence of such an argument, every believer is justified in taking it to be the case that God exists, and warranted just in the case that God’s non-existence hasn’t been established and we should expect to function the way we do in the case that God does exist.