On the basis of personal experience, self confidence develops and on the basis of self confidence, confidence in the Dharma develops on the basis of confidence in the Dharma, trust in the teacher develops. Teachers represent the Sangha and the Buddha is the origin of dharma. Thus faith and confidence in the efficacy of the three treasures of Buddha, Dharma and Sangha is established through practice. Unlike worldly accomplishments, genuine transformative practice and experience rarely come from reason or knowledge. For example, if a meditator is able to ease or even forget about leg pain, because they received proper instructions for meditating on pain, they will naturally develop confident confidence in themselves and their meditation practice. If a person just reads about meditating on pain, this experience is not actually going to alleviate that pain. However reasonable the method may sound, we have to practice it before we can form any opinion about its efficacy. Chan practitioners must have confidence in Buddha nature, or intrinsic freedom. But this confidence is not something we can rationalize. I've met Zen practitioners who in meditation, bring forth this faith in just sitting as an expression of awakening or Buddha nature. It is an expression of Buddha nature. But when I inquire further about it, I find that what they're doing is thinking about faith and conviction and Buddha nature while they sit there engaging in a monologue to remind themselves about their faith and conviction. This is not having confidence in Buddha nature, confidence is not thinking. It is a conviction that arises from experience. To develop confidence, we need to cultivate correct attitudes and use the methods of practice. When we practice with contentment, and interest, and move away from self reference, conviction, and our freedom naturally grows, the less self centered we are, the more our conviction grows. The more we learn to be okay with things as they are, be okay with adversity. Be okay with the ups and downs. The more we realize what our part is, what our job is, to do the groundwork bring our awareness to everything. Then as change happens, we do realize, oh, this really works. We may begin to realize we're a different person than we were. We go along successfully for a while our wife or husband or significant other may also notice a difference. That's a higher bar.