This quote from Napolean Hill has direct spiritual application.
If you don’t believe it yourself, don’t ask anyone else to do so.
It is virtually impossible not to transmit your doubts and insecurities to others through body language, tone of voice, inflection, word choice, and other subtle characteristics. When you show by your actions that you lack self-confidence, other people also begin to doubt your ability to perform. You can gain the respect and confidence of others. Begin by making a list of all the things you like about yourself and the things you would like to change. Make a conscious effort to build upon your positive strengths and correct your weaknesses. It may not be easy, but if you assess yourself objectively and persevere in your efforts, you will eventually prevail.
Hill applies this to one’s self-concept. We may apply it to the gospel. If we are not convinced of the truth of the message, it will come through to others. Confidence and certainty beget more of the same.