Why I defend all speech at all times

"Censorship is to art as lynching is to justice."
— Henry Louis Gates Jr.

With the printing press, Martin Luther liberated all of Europe from a corrupt system of paid pardons, compelled speech, and religious intolerance. But at the time, his protestations were considered dangerous heresy.

Martin Luther King Jr. liberated all of the United States from the stultifying effect of racism with broadcast speeches, articles, and pamphletary. But at the time, his protestations were considered "hate speech." Dangerous heresy, in other words.

To our ear, that sounds crazy. We know what hate speech is. We know what heresy is. It's the stuff we don't like, and there's no reason the stuff we don't like should be allowed in print. Right?

Here's the thing. Even the smartest human beings who ever lived were stupid about almost every single subject. Einstein didn't know two bits about a Turing Machine, and Turing couldn't make heads or tails of quantum physics. So can we really know for certain which are the heresies or which arguments have at least a grain of truth to them, especially if we allow the censors to block them from public scrutiny?