I agree with your point about the absurdity heuristic.

However, I'm not sure that the three following points are relevant to the case of theism:

• If a large number of intelligent people believe something, it deserves your attention. After you've studied it on its own terms, then you have a right to reject it. You could still be wrong, though.

• Even if you can think of a good reason why people might be biased towards the silly idea, thus explaining it away, your good reason may still be false.

• If someone cannot explain why something is not stupid to you over twenty minutes at a cafe, that doesn't mean it's stupid. It just means it's complicated, or they're not very good at explaining things.

What if we have strong evidence that the people who hold the seemingly absurd belief all have similar biases? More, what if a very large fraction of these people admit that they're biased, and are even proud of it?

That's exactly what the situation is with regard to theism, of course. Most theists admit that their religious beliefs are based only, or mostly, on faith. Some state it outright, others hide it behind circumlocutions and nebulous metaphors, and yet others need to be pushed a bit before they'll admit it, but the result is the same.

Does it still matter, then, that many of these people are intelligent, or that some of these religious beliefs may be very complex, or that I haven't studied some of them with great attention?