I've challenged the Faith Heuristic to a debate on whether faith is rational without evidence. This will be a four post debate, with me making the opening post.
Let's talk about beliefs. What is a belief? I define it as a statement that some person(s) rely upon to be true when deciding what action(s) to perform.
Now, let's think about trying to decide what to believe: We want our beliefs to be true, so that when we decide to perform some action to gain something we want the attempt will be successful.
If some proposition is not logically necessary, then that means that there are at least two propositions which might be the case. Since there are at least two propositions that might be true, then that means that neither (or none) of them has more than a 50% initial probability of being true (The initial probability means the probability of something being true before considering any of the specific observational evidence for that proposition). Suppose I that I just flipped a coin. The initial probability that the coin would land on heads is fifty percent, since we know from past experience that coins land on heads about 50% of the time and because if we assign both outcomes (heads and tails) equal probabilities, the odds of heads coming up will be 50%. The posterior probability that the coin landed on heads will be determined by my observation of how the coin flip turned out. If I observe it to be heads, then that raises the probability that the coin really did flip to heads to almost 100% (maybe not quite 100%, since there's a very small chance I could be hallucinating or something).
What I've just taught you is the basics of Bayes' Theorem. With Bayes' theorem, you figure out the initial probability of some hypothesis, and then you look at the specific observations which might help to confirm or disconfirm the hypothesis, and from that you can determine whether the hypothesis is probably correct or not. And by the way, in case you're wondering, Bayes' Theorem follows from simple logical principles, which I believe are laid out in the article I linked. Logical principles (Such as the Law of Identity: Things are what they are and are not what they are not) are necessary; They cannot be false and there are no other alternatives. So we need no evidence for them: We already know that they are 100% true.
Now let's look at faith without rational justification. Plenty of Christians have faith that their particular religion is true. But there are hundreds of religions, and billions more possible religions that no one has even thought of yet. As long as religion does not assert contradictory propositions (or can be modified so that it is logically consistent) then it is possible. There are billions of logically possible religions, and so the odds that a particular one of them is true is one out of many billions (I'll just say ten billion, for convenience). So the a priori probability that Christianity is true is one in ten billion. Now, the final probability could be raised or lowered if we only looked at some evidence. If there were many facts about the world which were much more likely on the assumption that Christianity is true than on the assumption that Christianity was false, then we could use Bayes' Theorem and these factors would make Christianity a great deal more probable. They might even show that Christianity is over 99% likely to be true.
But since Justin Martyr (The Faith Heuristic) is defending the idea that it is rational to have faith even without any evidence, then that means we don't have to bother with the posterior probability (i.e. "the evidence") that Christianity is true. We only have to see if it is rational to believe Christianity in light of the initial probability. And we've already seen that the initial probability for Christianity is very, very low. If you had a car that only started ten percent of the time, you would not think it reliable. What if you believed a religion that had even less of a chance of being true?
Christianity is therefore not a reliable belief because it is (initially, at least) probably false. Since our beliefs are the propositions we rely upon to be true, someone who believes Christianity without evidence is relying upon the unreliable. If this is not irrational, I do not know what is.