Karen Armstrong explains that the separation of religion from politics is a very modern idea.
One thing has to be made clear from the very, very beginning: up until the modern period, nobody had an idea of religion being something separate from other activities. Religion permeated all activities so that if you tried, for example, to take religion out of what we call politics, it would be like trying to take the gin out of the cocktail.
There’s no word for religion that corresponds in other languages to what we have come to mean by religion in the modern world. Words in Arabic like din, or in Sanskrit dharma, refer not to cult or religion but to an entire way of life. And the prophets of Israel had absolutely no time for people who separated religious observance from political concern. They lambasted people who said their prayers very nicely in the temple, but did not take any notice about the plight of the poor. And this is because oppression and injustice and human suffering are matters of sacred import.
So religion as we know it, this is something that was developed during the Enlightenment, when we separated religion and politics and created a secular state that separated the two. And that helped our modernisation, but up until about 1700 in Europe it would have been absolutely impossible to take religion out of politics at all. So because all states go to war – no state, however peace-loving it claims to be, can ever afford to disband its army – therefore, because religion permeated all these activities, therefore you have religion sort of … warfare was articulated or described in religious terms.