“If you don't believe in yourself, who will?' ~Maybeck”
Ridley Pearson (1953) American writer
Source: Disney after Dark
Attributed to Korda in The Power of Choice (2007) by Joyce Guccione, p. 52, the earliest occurrence of such phrasing yet located is by Martin Lawrence, in "What Up?" in Upscale : The Successful Black Magazine (February 1993), p. 79: "If you don't believe in yourself, then who will believe in you? The next man's way of getting there might not necessarily work for me, so I have to create my own ways of getting there."
Disputed
“If you don't believe in yourself, who will?' ~Maybeck”
Ridley Pearson (1953) American writer
Source: Disney after Dark
“Always, always, always believe in yourself, because if you don't, then who will, sweetie”
Marilyn Monroe (1926–1962) American actress, model, and singer
Variant: Keep trying, hold on, and always, always, always believe in yourself, because if you don't, then who will?
Aurelius Augustinus (354–430) early Christian theologian and philosopher
Earliest attribution found in Who Said That?: More than 2,500 Usable Quotes and Illustrations https://books.google.nl/books?id=7mn8AwAAQBAJ&pg=PT63 (1995) by George Sweeting. Online sources always attribute the quote to Augustine, but never specify in which of his works it is to be found. <br class="br">Disputed
Michel De Montaigne (1533–1592) (1533-1592) French-Occitan author, humanistic philosopher, statesman
Book III, Ch. 8
Attributed
“You cannot believe in God until you believe in yourself.”
Swami Vivekananda (1863–1902) Indian Hindu monk and phylosopher
Pearls of Wisdom
“Relationship Principle 5
Don't believe what anyone tells you about yourself.”
Sherry Argov (1977) American writer
Variant: Don't believe what anyone tells you about yourself.
Source: Why Men Marry Bitches: A Woman's Guide to Winning Her Man's Heart
“The first step is, if you don't believe it yourself your team darn sure won't.”
Rex Ryan (1962) American football coach
[Rex Ryan's press conference, http://espn.go.com/blog/new-york/jets/post/_/id/953/rex-ryans-press-conference-2, ESPN, McManus, Jane, August 6, 2010, http://www.webcitation.org/5x46RBB2S, March 9, 2011, March 9, 2011]