The mechanism isn't the same as for diseases[. . .] I suspect that, over time, individual selection favors those who are less zealous. The point is that a culture develops antibodies for the particular religions it co-exists with - attitudes and practices that make them less virulent.

Sometimes, reason [. . .] lets us see the blind spots, but not the dangerous memes. Either of these ways, reason can lead an individual to be unbalanced, no longer adapted to their memetic environment, and free to follow previously-dormant memes through to their logical conclusions.

This also has another side. If individual or cultural selection favors those who don't try to actually do what a religion or cultural norm tells them to do, what happens next depends on which variables are held constant. If the culture is constrained to hold constant the religion or cultural norms, then the resulting selection will cause the culture to develop blind spots, and also develop an unspoken (because unspeakable) but viciously enforced meta-norm of not seeing the blind spots. But if the culture is constrained to hold opposite meta-norms constant, such as a norm of seeing the blind spots or a norm of actually doing what one's religion or cultural norms tell one do do, then the resulting selection will act against the dangerous memes instead. This would make the culture safer for truth-seeking, and make the dividends of truth-seeking easier to pursue.