The Separation of Church and State in the Mediaeval Age and Today
Updated: Dec 21, 2022

What did the separation of church and state look like during the so-called "dark ages" – the mediaeval period? The answer is not straightforward. It is one of conflict and confusion.
To answer it, we need some background on the usage of the term "separation of church and state" as well as its historic development as a practice.
The Separation of Church and State
“The separation of church and state” may be the most misunderstood political phrase of all time. It's frequently invoked to support the removal of religious values from public life.

But is this usage accurate?
Interestingly, the term first appears in a letter by Thomas Jefferson, one of the American Founding Fathers, to the Danbury Baptists. In that letter, he reassures them that their right to practice Christianity would not be restricted by government legislation.
He references a "wall of separation" between the church and the state to illustrate his point.
Biblical Ideas: Two Kinds of "Church"
Arguably, the idea of the separation of Church and State originally comes from the Bible. Many Christians hold that the Bible recognises different unique “units” of government, including family government, civil (political) authority and the internal Christian Church structure.
There are two ways that Christians speak of the 'Church', and both have their roots in the Bible.
Firstly, in the New Testament, the Church is called Jesus’ Body and Bride. On this understanding, the Church is a worldwide collection of Christians – of those who follow Jesus.
The second understanding of 'Church' is not directly found in Scripture. It is a later invention deduced from the pattern of Church leadership laid out in the New Testament. According to this usage of the word, the Church is an institution or a corporate, physical manifestation of the worldwide Church.
The second meaning of the word is the one that is dominant today (particularly in secular circles). This is the understanding we are drawing on when we speak of the church building as 'Church' and when we say that we are 'going to Church'. It is also the concept in mind when we refer to different denominations – i.e., the 'Catholic Church', the 'Episcopalian Church', etc. – as 'Churches'.
It appears that it is also this second definition of the word 'Church' that is used in the phrase, "the separation of church and state". Hence, the term was originally referring to a separation between the institutions of the Church and the State. This is consistent with the Biblical understanding of the separation.
Similarly, the separation Thomas Jefferson speaks of addresses the association between the institutions of Church and State, not between Christians and the Government.
Conflict Between Church and State: A History

No particular event can claim full credit for bringing about the famed church and state conflict. However, the most significant was probably the AD 380 Edict of Thessalonica, when emperors Theodosius I, Gratian, and Valentinian II made Christianity the Roman Empire’s state religion. <