Vox Populi Vox Dei

A major transistion in theory of governance has taken place. We have transistioned from a theory of goverment that held that goverment was acceptable because it was the will of god or the gods and that right flowed from them to one where goverment is only legitimate if it is by consent of the governed. Note that this does not neccesarily mean democracy merely consent.
We can see this in the langauge. We go from the "divine right of kings", "mandate of heaven", "vox dei vox populi" to the phrase "the will of the people".

This change in phrases reflects the belief, as expounded by Locke, that people have the right and the responsibility to govern. His is not an argument that they would be the best governors - but that it is their duty to so choose their goverment. Some things in life you have to do yourself and according to this view goverment is one of them.
This has changed everything. Religion is no longer worth controlling because power does not flow from it. Freedom of religion then becomes inevitable. Religious wars have all but ceased where this tenet is accepted. Instead that energy is now channeled to controlling the hearts and minds of the people - manufacturing consent if you will. This is done either through the soft persuasion of a political campaign and power of political networks or the hard persuasion of secret police, information control, and gulags. Both stalin and hitler accepted this tenet of Locke's. That is why their regimes were both so concerned with controlling what their people were thinking and they both acted in similar ways even though their philosophies were wildly different in other ways.

The first order of business for all rulers is to stay in power. If rulers will not manufacture the neccesary consent then rest assured another will.