Speak in Deep Wisdom. Choose the things you would like them to believe, such as "form beliefs based on evidence", and make it sound like a beautiful poetic truth.

This borders dangerously on Dark Arts, though. If someone is capable of better forms of reasoning, then only use the best they're capable of, even if a more flawed argument could get them to the right conclusion. And, of course, try to convey the most meta ideas you can -- "use evidence", not "disbelieve religion".