It’s true that religion has often served to unite people against tyranny, as well as to justify it. But in many cases, when a religious rebellion overcame a tyrant, it was only to install a different tyrant whose beliefs matched those of the revolutionaries. Christians were at first ruthlessly persecuted by the Roman Empire, but when they ascended to power, they in turn banned all the pagan religions that had previously persecuted them. Protestant reformers like John Calvin broke away from the decrees of the Pope, but Calvinists created their own theocratic city-states where their will would reign supreme.
Similarly, when King Henry VIII split England away from the Catholic church, it wasn’t so he could create a utopia of religious liberty; it was so he could create a theocracy where his preferred beliefs, rather than the Vatican’s, would be the law of the land. And in just the same way, when the Puritans fled England and migrated to the New World, it wasn’t to uphold religious tolerance; it was to impose their beliefs, rather than the Church of England’s.
It’s only within the last few centuries, in the era of the Enlightenment, that a few fearless thinkers argued that the people should govern themselves, that society should be steered by the democratic will rather than the whims of an absolute ruler. The kings and emperors battled ferociously to stamp this idea out, but it took root and spread in spite of them. In historical terms, democracy is a young idea, and human civilization is still reverberating from it — as we see in autocratic Arab societies convulsed with revolution, or Chinese citizens rising up against the state, or even in America, with protesters marching in the streets against a resurgence of oligarchy.