Of the Big Three, Free Speech is the one that supports all the others. My unaLIENable right to speak my mind, and your unaLIENable right to respond is the basis of a free and open society.
The two greatest enemies of Free Speech are the State and Religion. The most dangerous condition is when Religion has sway over the State. Throughout history, the most repressive regimes have all been theocracies. The first thing a theocracy does is shut down criticism of itself, then it proceeds to define everything in terms of itself. Since Religion claims to have the keys to all of existence, naturally it sees just about anything as a threat to its grandiose claims - especially the right to freely speak the contradictions and pure fantasy inherent in all religions.
In ancient Rome, it was a capital crime to speak out against Caesar, who being deified as a matter of course by the Senate, was a demi-god created by the State. To speak against Caesar was to speak again Nature and the Universe themselves.
In its most benign form, theocracies are still intolerant of Free Speech. Thailand does not tolerate any speech about the king, unless it is glowing and praiseful. One can get in quite a bit of trouble for pointing out flaws, contradictions or wrongs on the part of the king, and to a certain extent the immediate royal family, as well.
Here in Indonesia, there is a blasphemy law that primarily protects Islam, though it has occasionally been used by Hindus, Buddhists and Christians. Most recently, Jakarta governor Ahok quoted a bit of the Koran that prohibits Koranic verses from being used for political purposes. Of course, he was immediately charged with blasphemy (being a Christian and reciting the Koran), and is still being politically attacked for reminding folks what the Good Book says.
The US is a rather strange situation. Since the government and Religion are not allowed to be bedfellows, a sort of secular religion was created. Blending "science", "environmentalism", "tolerance", and "diversity", a beast called Political Correctness was born that has the same effect as Religion, but the word "religion" is religiously avoided. Through the rampant use of PC, people are forbidden to raise topics such as immigration, climate change, or any of a host of other topics that "blaspheme" the State Religion.
In any repressive regime, there are always quasi-official gangs whose job it is to go round and commit acts of violence and destruction against those deemed to be enemies of the Religion. Here, it;s the FPI, in the US it's SJW. Either way, the goals and results are the same. It's just a game of semantics, really.
The Roman Church went on its own pogrom, known affectionately as the Inquisition, in order to stifle Free Speech. People like Giordano Bruno and Galileo Galilei were hounded and harassed, and in some cased killed, for daring to speak out of turn. Even up until the 50s, publishers strove to receive a "Nihil Obstat" and "Imprimatur" from the Church so that sales would not be limited by having the Church ban its mind-slaves from reading the books. Even though the system was a sanctioned form of bribery much like "halal" or "kosher", it still had the effect of stifling Free Speech at some level.
The fact of the matter is that limiting Free Speech is a very old institution used primarily by the state and religious authorities. It is a blatant admission of weakness, since those enforcing limits know that their "arguments" are so inherently illogical, they cannot stand up to open inquiry and criticism. Speech limits always carry the threat of violence, and an individual or group who resort to violence have run out of verbal defenses of their positions and admitted their weakness through the use of force.
Of all the perceived sins against the State and Religion (i.e. authority), Free Speech is the most dangerous, since any limit on the individual must be backed up with the threat of violence. Furthermore, "authority" will continuously expand its definition of limits until it meets resistance from the greatest number of people. and ultimately those threatened will expand until it includes everyone who is not an "authority".
What most people fear about Free Speech is that eventually they will run into ideas that offend them. For speech to be truly free, we cannot limit anyone's right to express ideas or opinions, no matter how ridiculous or offensive they are. Once a group accepts that there is any limit on speech, then all speech is limited, since the movement will ultimately expand to encompass just about everything except the proscribed "right think".
Most people will agree that the Nazis were very bad folks and that the rest of us don't want them around. However, we are forced to admit that even the Nazis cam up with ideas like the Volkswagen Beetle. ao
In every ugly or abhorrent idea, there is some kernel of truth that can be gleaobned. We may not agree with, or may even be deeply offended by someone's speech, but it is absolutely vital that we allow that speech. By limiting anyone's speech, we do ourselves and the world a disfavor by cutting off potentially world-changing ideas, no matter how buried in hurtful or hateful trappings.
It is never speech that hurts anyone, it is the actions taken based on it, and that is where limits are properly placed.
Galileo was imprisoned for life by "authorities" because he observed objects orbiting a body other than the Earth. This was deeply offensive the the State and Religion. However, imprisoning him for life did not change the facts, only who could hear them.