“If you only believe when it's easy, you don't really believe.”

Ender's Game series, Shadow of the Hegemon (2001)

Edie : Girl On Fire (2006)
Context: But I really, since I exist, at all, I believe that it's possible for people... I've lived through impossible situations. So I believe in it. I just believe, and that's the magic... That's the whole thing, you talk about magic that there's to believe in, and it is there. But most people don't really believe in it. And I refuse, like, since I'm still alive and done the things I've done and seen things and understood things as far as I have, and I am alive, I mean physically intact. When I shouldn't be, according to medical reports and so forth. I mean I should be, not here. That's all there is to it. So the magic's working and it's a rare situation.

The Post Office Girl (published posthumously in 1982)

O interview (2003)
Context: I want to direct a movie in Mexico, in Spanish. The story is about how when we're really young, our dreams are colorful and big and abstract and interesting and imaginative. As the realities of life hit, our dreams become so common. To dream big doesn't necessarily mean to imagine becoming the biggest movie star in the world. Dreaming big is about taking the simplest thing in life and enjoying it — and seeing it as the biggest thing that can possibly exist. … I work in an industry that is the first to kill this ability because everything is so celebrity oriented. I am part of a cancer. In my world, you have to be so beautiful, so skinny, so rich, so famous — and I don't believe you really have to be any of those things. You simply have to be who you are.

Podcast Series 1 Episode 1
On Monkeys

Academy of Achievement interview (2006)

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

I Don't Believe You, written by Pink and Max Martin
Song lyrics, Funhouse (2008)