“You cannot believe in God until you believe in yourself.”

Source: Tiny Beautiful Things: Advice on Love and Life from Dear Sugar

Source: Paris France (1940), p. 56

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

Patheos, The Cow http://www.patheos.com/blogs/reasonadvocates/2016/01/22/the-cow/ (January 22, 2016)

Source: The Witch of Portobello (2007), p. 152.
Context: You are what you believe yourself to be.
Don't be like those people who believe in "positive thinking" and tell themselves that they're loved and strong and capable. You don't need to do that because you know it already. And when you doubt it — which happens, I think, quite often at this stage of evolution — do as I suggested. Instead of trying to prove that you're better than you think, just laugh. Laugh at your worries and insecurities. View your anxieties with humor. It will be difficult at first, but you'll gradually get used to it. Now go back and meet all those people who think you know everything. Convince yourself that they're right, because we all know everything, it's merely a question of believing.
Believe.

Source: In Search of Satisfaction

Source: Dracula