If you could choose/write one sentence to explain the value of free speech to the people of the world who don't get it...

... what would it be? Lawprof Stephen L. Carter — addressing the anti-blasphemy mobs — says:

The best statement of our constitutional rule remains the one announced by the U.S. Supreme Court 40 years ago in Police Department of the City of Chicago v. Mosley: “To permit the continued building of our politics and culture, and to assure self-fulfillment for each individual, our people are guaranteed the right to express any thought, free from government censorship.”

If that's the best statement, we are in trouble. I love the idea of looking for a short, apt sentence saying why we believe in protecting freedom of speech, but surely there must be hundreds of better statements than that.

To assure self-fulfillment for each individual... I think it's creepy for the government to even purport to assure self-fulfillment. You can have all the freedom in the world and it won't assure much of anything. And self-fulfillment... you have to already have a particular view of the meaning of life before you put self-fulfillment at the center of the universe. And if you're trying to convince people who begin with a conviction that God is the center, you're blocking your own pathway into their minds. (And they already want that pathway blocked.)