It's funny you refer to the tyranny of Christianity when by far the most tyrannical regimes were all officially atheist (Stalin's, Mao's, Pol Pot's, North Korea), and the west has been getting steadily more and more authoritarian as it's gotten less and less Christian. Fundamentally most non-religious people are utilitarians, who see no inherent value in a life and hence are open to the idea of sacrificing a few lives for the "good of the many".
The first step to dictatorship is convincing people there's no such thing as "absolute" rights, so that they won't object to the regime making up more and more excuses to violate people's rights. And without a belief in some kind of higher power the majority of people also won't have any reason to believe that some rights are inherently inviolable.