Steven Nadler in the New York Times:
Spinoza approaches the issue of individual liberty from several perspectives. To begin with, there is the question of belief, and especially the state’s tolerance of the beliefs of its citizens. Spinoza argues that all individuals are to be absolutely free and unimpeded in their beliefs, by right and in fact. “It is impossible for the mind to be completely under another’s control; for no one is able to transfer to another his natural right or faculty to reason freely and to form his own judgment on any matters whatsoever, nor can he be compelled to do so.”
For this reason, any effort on the government’s part to rule over the beliefs and opinions of citizens is bound to fail, and will ultimately serve to undermine its own authority. A sovereign is certainly free to try and limit what people think, but the result of such a policy, Spinoza predicts, would be only to create resentment and opposition to its rule.
It can be argued that the state’s tolerance of individual belief is not a difficult issue. As Spinoza points out, it is “impossible” for a person’s mind to be under another’s control, and this is a necessary reality that any government must accept. The more difficult case, the true test of a regime’s commitment to toleration, concerns the liberty of citizens to express those beliefs, either in speech or in writing. And here Spinoza goes further than anyone else of his time: “Utter failure,” he says, “will attend any attempt in a commonwealth to force men to speak only as prescribed by the sovereign despite their different and opposing opinions … The most tyrannical government will be one where the individual is denied the freedom to express and to communicate to others what he thinks, and a moderate government is one where this freedom is granted to every man.”