To-day I learned about two moments in European History, the peace of Augsburg (1555) and the peace of Westphalia (1648).
The Treaty of Augsburg was signed between the Holy Roman Empire and a group of Lutheran princes who sought to break away from the Roman Church. An important phrase in the treaty was cuius regio, eius religio – “whose realm, his religion”. This established the principle amongst the states of the HRE that a ruler's religion would determine the religion of that state. The two choices being Lutheranism and Catholicism.
The conversion of more Romans to Lutheranism within the HRE and the conversion of some to Calvinism created tensions that caused the treaty to collapse and led to the Thirty Years' War (1618 to 1648). This was a disastrous conflict that engulfed much of Europe and led to millions of deaths. Peace was eventually restored in the form of two new treaties referred to as the peace of Westphalia in 1648.
The peace of Westphalia effectively invented the modern nation state. It granted full sovereignty to secular rulers. It established borders within which the ruler was the sole authority, with freedom to choose between Roman, Lutheran, and Calvinist versions of Christianity. In effect, this was the origin of the idea of a modern nation state.
Pope Innocent X was very angry about this, and he called the treaty "null, void, invalid, iniquitous, unjust, damnable, reprobate, inane, empty of meaning and effect for all time". But by this point, secular rulers were powerful enough ignore him.
There was an unintended consequence. Despite allowing for freedom of worship to minorities, in fact, religious tolerance decreased in most places. Persecution of Anabaptists in Germany, for example, grew in ferocity. Heretics were now enemies of the state, since they pledged allegiance to an authority other than the state. They were tortured, murdered, or driven out. The Netherlands did provide some relief as they were tolerant, but eventually many groups such as the Amish, Mennonites, and the Plymouth Brethren left Europe for the new colonies in the Americas.
America filled up with radical religious refugees; early on German was the most widely spoken language. Some, like the Amish were harmless to outsiders and simply wanted to be left alone to practice their faith. Others were more militant and wanted power. The English Church was also well represented. None of them respected the sovereignty of the indigenous people or even recognised them as human. Genocide and mass expropriation followed. And then millions of slaves were imported from Africa to work the land.
When the US constitution was drafted, it was important to seal off government from the influence of any one Church. If one religious group gained power, they were likely to set off another round of bloody persecution of other groups as seen in Europe. The US became the first nation state to separate church and state. France soon followed suit. The legal separation of church and state is not exactly secularism in the US. It was self-preservation.
Fundamentalist Christianity is still very popular in the US and very influential. Waves of immigration from Ireland and Italy brought many Roman Catholic Christians. Moreover, America spawned its own religious cult in the form of Mormonism. Other Christian inspired cults soon followed though few ever reached the proportions of the Mormonism. Separation of church and state is even more important now with these historically aggressive and intolerant religions in close proximity.