If it is decided in advance that there are things that it's illegal to say, you can't have a free debate, and without a free debate, democracy cannot be sustained.
In the aftermath of 9/11 and other terrorist acts, some democratic countries have passed laws that in reality reintroduce censorship. Britain for example has introduced its Terrorism Act 2006, which forbids the "glorification" of terrorism.
In China, which is not a democracy - and where the government is fighting tooth and nail for it not to be - there is extensive censorship and always has been. Its censorship of the Internet is one of the most advanced in the world. China has a well-educated and inquisitive population and a long tradition of a solid, legally based society, so the regime's censorship is probably the major barrier to China becoming a democracy.
Next chapter: Serious Adversity