“If you set to work to believe everything, you will tire out the believing-muscles of your mind, and then you'll be so weak you won't be able to believe the simplest true things.”

Source: Burned

Variant: Now... if you trust in yourself... and believe in your dreams... and follow your star... you'll still get beaten by people who spenttime working hard and learning things and weren't so lazy. Goodbye.
Source: The Wee Free Men

God doesn't believe in atheists (2002)

You and Your Research (1986)
Context: Most people like to believe something is or is not true. Great scientists tolerate ambiguity very well. They believe the theory enough to go ahead; they doubt it enough to notice the errors and faults so they can step forward and create the new replacement theory. If you believe too much you'll never notice the flaws; if you doubt too much you won't get started. It requires a lovely balance.

1840s, Essays: First Series (1841), Self-Reliance
Context: I read the other day some verses written by an eminent painter which were original and not conventional. The soul always hears an admonition in such lines, let the subject be what it may. The sentiment they instil is of more value than any thought they may contain. To believe your own thought, to believe that what is true for you in your private heart is true for all men, — that is genius. Speak your latent conviction, and it shall be the universal sense; for the inmost in due time becomes the outmost, — and our first thought is rendered back to us by the trumpets of the Last Judgment.

Source: Someone Like You

Pope Urban VIII on Private Revelation http://www.medjugorje.org/purban.htm (2001).
Whether or not Urban VIII said this is debated. He did make a public statement about private revelations and their dissemination in the Catholic Church in his Constitution, Sanctissimus Dominus Noster of 1625-03-13.
Misattributed quotes