“Have the courage to have your wisdom regarded as stupidity.”

Source: The Road Less Traveled: A New Psychology of Love, Traditional Values, and Spiritual Growth

As quoted in Teen Ink : What Matters (2003) by Stephanie H. Meyer, John Meyer, and Peggy Veljkovic, p. 309

Source: His Dark Materials, The Amber Spyglass (2000), Ch. 36 : The Broken Arrow

What is Enlightenment? (1784)
Variant: Have the courage to use your own reason- That is the motto of enlightenment.
Source: Groundwork of the Metaphysics of Morals
Context: Enlightenment is man’s leaving his self-caused immaturity. Immaturity is the incapacity to use one's intelligence without the guidance of another. Such immaturity is self-caused if it is not caused by lack of intelligence, but by lack of determination and courage to use one's intelligence without being guided by another. Sapere Aude! Have the courage to use your own intelligence! is therefore the motto of the enlightenment.

"To the Indianapolis Clergy." The Iconoclast (Indianapolis, IN) (1883)
Context: The inventor of paper—and he was not a Christian—did more than all the early fathers for mankind. The inventors of plows, of sickles, of cradles, of reapers; the inventors of wagons, coaches, locomotives; the inventors of skiffs, sail-vessels, steamships; the men who have made looms—in short, the inventors of all useful things—they are the civilizers taken in connection with the great thinkers, the poets, the musicians, the actors, the painters, the sculptors. The men who have invented the useful, and the men who have made the useful beautiful, are the real civilizers of mankind. The priests, in all ages, have been hindrances—stumbling-blocks. They have prevented man from using his reason. They have told ghost stories to courage until courage became fear. They have done all in their power to keep men from growing intellectually, to keep the world in a state of childhood, that they themselves might be deemed great and good and wise. They have always known that their reputation for wisdom depended upon the ignorance of the people.

Source: In the Drift (1985), Chapter 1, “Mummer Kiss” (p. 4)