For one investigating the early signs in favor of the concept of separation of Church and State, John Guy's excellent critical study of the the life and death of England's Archbishop of Canterbury, Thomas Becket is a fruitful beginning. The story of Becket has fascinated people for centuries because it is the story of meritocracy, the rise of a middle class Londoner to the post of Chancellor, very close to the King of England and then to the highest post in Catholic England, Archbishop at Canterbury. His story is infamous because his murder takes place in the Cathedral.
The politics of Henry II are intricate, however, he himself is bald in his grab for land and power. As a king of divine right, he tests the limits of his control of the Church by expropriating lands, monies and appointments. Christendom at his time is split in controversy with a competing anti-pope so that the papacy of Rome must deal delicately with Henry constantly courting his fealty and support.
Becket is complex, initially seen as a loyal subject of the king but clearly transforms himself into a penitent loyal to the Church. He surprises Henry with his lectures of Church autonomy and the limits to what a king can and cannot do.
One sees the waffling of a papacy instead of steadfast support of Becket's position because the Pope does not want to alienate the powerful King of England.
I enjoyed Mr. Guy's critical eye in his evaluation of the hagiography, able to see hyperbole and sift through the sources to find the truth about the main characters of this episode in England's history.