I would rather emphasize Raising the Sanity Waterline. If we bar theists outright, we miss the opportunity to discuss rationality with them in different contexts. We don't get to learn what insights they might have when their biases are not in the way. We don't get to teach them about all the biases using nonreligious examples, so that they might, on their own, figure out to check for those same biases in their theistic beliefs. If we allow theists, we still have the karma system to bury any obnoxious comments they make on discussions of religion, and the same karma system will encourage them to participate in the areas where they will get the most benifet.