This is a little long, but very interesting.
The scripture for today, December 27 (12/27), is 1st Kings 12:27 as found in the Old Testament of the Bible:
For centuries and millenniums, kings selected a patron god for their nation, a god they most revered to protect them. The patron god was selected by the king. Nations were always ruled by either a high priest or king. Some nations even had priest-kings.
So, when Constantine conquered much of Europe and set up his empire, he needed a patron god. One day during a battle, he looked up and saw a cloud formation that reminded him of things he’d heard about Jesus. So he made Jesus his patron God, and Christianity his empire’s required religion. He even rode his army through rivers, then declared them baptized. He ordered conquered peoples to be baptized or killed. Headquarters was Rome.
Luther was friends of the Duke [king] of Germany, so when he broke off from Rome, the official state religion became Lutheranism. John Calvin was in the government of Switzerland, so when it broke off from Rome, the official state religion became Calvinism. When the king of England broke off from Rome, he became head of the Church of England (and Prince Charles is head of the Church there today). John Knox was influential with the parliament of Scotland, so his Presbyterianism became the official religion of Scotland.
In each instance, the king or parliament could force its citizens to follow the chosen religion of that nation. If people didn’t, they could legally put them in prison, torture them, and execute them at will. And sometimes they did just that.
Eventually, the idea of separating religion from the state government became popular. And that is where many nations are today. But it does not mean people in government cannot express religion. It just means they cannot force citizens to be a particular government-selected religion or be in danger of punishment. That is true “separation of church and state”.