Rationality is to a Christian somewhat as the Dark Arts are to us. Christians have often made conversions based on reason, even though giving reason legitimacy makes their converts "dumber" and less-able to resist the temptation of reason.

They haven't said "these practices are off-limits to us". They strive for an optimal tradeoff between winning converts and corrupting their religion. We can consider their policies to have been selected by evolution. So we should be suspicious of claims that we, using reason, can find tradeoffs better than 2000 years of cultural evolution can. Particularly when our tradeoff ax + by involves suspicious numbers like a=0 and b=1.