Last Sunday, as lovers of free speech filled the streets of Paris, lovers of elegant English period soap opera were watching “Downton Abbey” in America. The episode featured a frisky Lady Mary, in 1924, asking her maid to purchase a contraceptive device for her in advance of a hookup with Lord Gillingham. She pointed to a section in a book, likely one of the pioneering sex manuals of the British botanist Marie Stopes, for guidance.

“Married Love,” first published by Dr. Stopes in 1918, was banned as obscene in the United States until 1931. Why? Because, you can’t just have women empowered with information about the joys of sex. Next thing you know, they’ll start liking it. Ignorance is not bliss; it’s controlling.

A cartoon is harmless. It may be crude, offensive or obscene . but that’s the point. Knowledge is something else, something to be feared. And so the United States, that cradle of unfettered speech, for more than decade outlawed the work of a scientist writing about the informed delights of female sexuality.

We laugh at the prudes with power from another era. But they are with us still. As much of the world experiences a pivotal moment on free speech, it’s worth remembering how we got here . and why it will take a while for the rest of the world to sort it through.

A cleric in Saudi Arabia recently proclaimed that building a snowman (or woman) was blasphemous. In that same country, a blogger was lashed for preaching tolerance. The orthodox newspaper in Israel, HaMevaser, scrubbed women from the photo of the Paris unity rally . gone, erased, so as not to offend.

And in our own country, which once criminalized the poems of Allen Ginsberg, the novels of Henry Miller and the sensible sex advice of Dr. Stopes, Mike Huckabee has attacked the Obamas’ parenting because they allow their children to listen to Beyonce. The singer’s lyrics are “toxic mental poison,” he said. This is very curious, even for Huckabee, who has made millions promoting himself as a victimized rube. Huckabee has performed, with Ted Nugent, the song “Cat Scratch Fever.” In his world, it’s O.K. for a middle-aged man to crow about pubescent promiscuity, but not a young woman.

Sex is at the root of most free speech fights. Whatever it is that incites lust, it also incites the censorious. Religion is not far behind. You could argue that the two are twined, and it would be hard to rebut. The simple genius of the First Amendment ties free speech to protection from a state religion . in the same sentence. There could be no real free expression, the American founders recognized, in a nation with a state-sanctioned religion.

This freedom, by its very definition, sets up a serious conflict in the interconnected global village. Protected speech in this country is blasphemous in another. Satire, in the form of silly Seth Rogen movie, is a provocation prompting a cyberwar in the closed society of North Korea.

China, rapidly trying to catch up with Western democracies in all things monetary and technological, is medieval in the realm of free expression. This week, China’s state news agency, Xinhua, ran a piece by its Paris bureau chief, who wrote that “unfettered and unprincipled satire, humiliation and free speech are not acceptable.”

But even in countries where speech is protected, that freedom is suspect. The Washington Post quoted Nasser Lajili, a French city councilor and a Muslim, who condemned the attacks in Paris with a caveat: “But I think freedom of speech needs to stop when it harms the dignity of someone else.” That same attitude has found a home in American universities, who fear Bill Maher, or the sex columnist Dan Savage.

Pope Francis, a voice of reason and progressive thought on most things, took a big step backward Thursday with his comments on expression. “You cannot provoke,” he said. “You cannot insult the faith of others. You cannot make fun of the faith of others.”

In fact, you can. Maybe you shouldn’t. Maybe such provocations are in poor taste, or degrading. Yet an enlightened society should be able to take the punch of satire and ridicule, even coarse satire and savage ridicule. It’s an evolving construct, to be sure, and may never find favor in the majority of the world’s countries.

It still has trouble finding favor in our country. Donald Trump, whose daily musings are proof that nothing is more revealing than a Twitter account in the hands of a simpleton, compared Charlie Hebdo to “rag magazine Spy that was very dishonest and nasty and went bankrupt.” He was referring to Spy, the thinking person’s Mad magazine. Or take a look at a list of banned books in the United States over the last hundred years: It’s a guide to good literature, and the small minds behind American hypocrisies.

True free speech is a radical idea. But at least all nations should agree that free expression is never a reason to kill. A faith that cannot withstand ridicule is no faith at all. And a faith that cannot laugh at itself is a faith that defies human nature.

“These people who came to kill us . they are fanatics and assassins,” said the Paris cartoonist Renald Luzier, who drew the Charlie Hebdo cover of a weeping Muhammad this week. “But above all, they are people who lack a sense of humor.”