Here’s an interesting text from William Walker Atkinson:

“”“Faith” Is a word that has been often misused, misapplied and misunderstood. To many it means simply that attitude of mind which will accept anything that is told it, merely because someone else has said it—credulity, in fact. But those who have penetrated within the shell of the word know that it means something far more real than this—something imbedded deep down in the Heart of Things. To those who understand the LAW, Faith is the trolley-pole which one raises to meet the Great Forces of Life and Nature, and by means of which one receives the inflow of the Power which is behind, and in all things, and is enabled to apply that Power to the running of his own affairs.

Faith in oneself is of primary importance, for unless one has it he can never accomplish anything; can never influence any other person’s opinion of him; can never attract to himself the things, persons and circumstances necessary for his welfare.

A man must first learn to believe in himself before he will be able to make others believe in him. People are prone to take a person at his own estimate. If one is weak, negative and lacking in self-confidence, he surrounds himself with an atmosphere of negativity which unfavorably impresses those with whom he comes in contact. If one be strong, confident and positive, he radiates like qualities, and those coming in contact with him receive an impression of these qualities. The world believes in those who believe in themselves. And so you see it is of the utmost importance to you that you cultivate this Faith in yourself.

And not only does Faith in yourself operate in the direction of influencing others with whom you come in contact, but it also has a most positive bearing upon your own mental status and thoughts.

If you deaden your mind with a negative attitude toward yourself, you stifle budding ideas, thoughts and plans—you choke the budding plants of your mentality. But, if you let pour forth a full, abiding, confident Faith in yourself—your abilities, your qualities, your latent powers, your desires, your plans—your Success, in short—you will find that the whole mental garden responds to the stimulating influence; and ideas, thoughts, plans and other mental flowers will spring up rapidly. There is nothing so stimulating as a strong, positive “I Can and I Will” attitude toward oneself.

And you remember what has been said about the Law of Attraction—you remember how “like attracts like,” and how one’s Mental Attitude tends to draw toward him the things in harmony with his thoughts. Well, this being so, can you not see that a Mental Attitude of Faith or Confidence in Oneself is calculated to attract to you that which fits in with such Faith—that will tend to materialize your ideal?””

Naturally, it’s not just saying “believe in yourself” as general advice. We go in-depth how you can actually do it in the course:
>>> 10 Steps to Inner Power