Some people think church and state should be separate. 
Some people believe church and state should be separate. 

Some people believe the church and the state should be separate. This idea means religion and government should stay apart. 

The separation of church and state is a big idea about how much distance should exist between religion and government. 
This idea has a very long history. In ancient times, religion and the state were often closely linked together. For example, in ancient Athens, Socrates was convicted for impiety. Later, a thinker named St. Augustine wrote about this in his book, "The City of God." He believed the earthly city and the heavenly city had many points of overlap. He thought the earthly city should help make the heavenly city possible on earth. 
New ideas began to grow during the Reformation. Martin Luther spoke about having two different kingdoms. This helped start modern ideas about separation. Some groups, like the Anabaptists, took this even further. A man named Michael Sattler argued that these two kingdoms should stay completely separate. He believed religious believers should not hold public office or vote. This was meant to protect the church from the power of the state. In England, King Henry VIII broke away from the Catholic Church in 1534. He made himself the ruler of the Church of England. This meant the monarch kept authority over the church for a long time.
Philosophers during the Enlightenment helped spread these ideas to more people. 
Thomas Jefferson is a key person in this story. 

The separation of church and state is a philosophical and legal concept. It defines the political distance between religious organizations and the state. This concept involves two main ideas: creating a secular state and disestablishment. A secular state is a government that is not officially tied to a religion. Disestablishment is the process of changing an existing, formal relationship between a church and the state.
Different societies manage this relationship in many unique ways. Some use the arm's length principle to keep both entities independent. In France, the government uses a strict secular principle called laïcité. Other nations, like Denmark and England, maintain an official state church. Some countries follow a policy of accommodationism, where religious symbols appear in public. In practice, separationism can range from total separation to having a state religion.
This idea has roots in antiquity, when religion and the state were often linked. In ancient Athens, the connection between the two led to the conviction of Socrates for impiety. Later, St. Augustine explored this relationship in his work, "The City of God." He examined the overlap between the "earthly city" and the "city of God." Augustine believed the temporal city should help make a heavenly city possible on earth. 
During the Middle Ages, power struggles shaped how these two spheres interacted. Many monarchs ruled by the idea of divine right. This theory, known as caesaropapism, suggested a king ruled both his kingdom and the church. Meanwhile, the Catholic doctrine argued the Pope held ultimate authority over both. These conflicting views led to many crises of leadership. One example was the Investiture Controversy, which was resolved by the Concordat of Worms in 1122. 
The Protestant Reformation brought new ways of thinking about these two kingdoms. Martin Luther articulated a doctrine involving two separate kingdoms. This idea helped start the modern concept of separation. The Anabaptists, a group in the Radical Reformation, took this idea further. Michael Sattler argued that these two kingdoms must remain completely separate. He believed religious believers should not vote or hold public office. This was intended to protect the church from state power.
During the Enlightenment, philosophers provided systematic arguments for religious liberty. John Locke was a major influence on this movement. 
In the United States, Thomas Jefferson helped define this concept through his writing. 

Today, these philosophies are connected to many broader social ideas. They relate to secularism, religious liberty, and religious pluralism. In many European states, these ideas led to the creation of the welfare state. This shift allowed the state to take over some social roles once held by the church. This transition helped create a more culturally secular public sphere. These legal structures continue to define how organized religion and the state interact in the modern world.
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