I find it's helpful to pitch rationality by tying it to reason. "Reason" is something that secular people feel warm fuzzies around, and so starting from what they know and like works well. From reason, it's easy to transition toward what it would be reasonable to do, namely be reasonable about how our minds work and how we should improve them. For example, check out this video of a presentation I made as a Rationality 101 for Secular People.