...disgustingly heinous and depraved nature of what happens when The Church and the State are one and the same (as in the beginnings of this mess we are currently still in), the concept would likely never have occurred to anyone as being necessary. Imagine -- no religion.
The beginnings of this concept in fact go back to the time of the Israelites where various kings attempted to control the influence of the Temple priests. The wars fought all during Medieval times furthered the idea among many of its necessity. The Church with its intrigues, power-struggles and back-stabbing, are at the core of much of that upheaval and death. And not to overlook their Inquisitions, where "heretics" who often seemed to have a lot of wealth, were shaken-down to buy their innocence. And the uncooperative ones being found guilty and having their possessions seized by The Church. And now their plunder adorns the walls of the Vatican.
Later,
http://en.wikipedia.org/wiki/Separation_of_church_and_state_in_the_United_States">John Locke wrote that "
the government lacked authority in the realm of "individual conscience," as this was something rational people could not cede to the government for it or others to control."
As for
http://en.wikipedia.org/wiki/Mary_Dyer">Mary Dyer, she understandably sought a separation of church control of the state, as her battle and disagreement with them was of a theological one. And one for which the punishment was death. Her belief that god speaks directly to people instead of "just" through the clergy, is the central reason for her position of support of separation. Because she and Anne Hutchinson were a direct threat to the Patriarchal Authority (a battle that continues to this day). And she is certainly to be remembered for her contribution.
However, it was Thomas Jefferson in his
http://en.wikipedia.org/wiki/Separation_of_church_and_state_in_the_United_States">Danbury Baptist letters, where the concept is made crystal clear:
"Believing with you that religion is a matter which lies solely between Man & his God, that he owes account to none other for his faith or his worship, that the legitimate powers of government reach actions only, & not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should "make no law respecting an establishment of religion, or prohibiting the free exercise thereof," thus building a wall of separation between Church & State."
- So from looking at the historical record, it would seem that pulling the priests away from the levers of power has been something we've had to do for sometime. Because we know what happens when they obtain power and control over people's lives. They've demonstrated this over and over again. And its never good....