And even today Shiah Muslims fight Sunni Muslims in various part of Muslim world, Hindus fight Muslims in India, Protestants fight Catholic in Ireland and even in Japan, fanatics of the Japanese religious cult of Doomsday spread poisonous gas in Tokyo underground railway station as a religious ritual.
If one defines religion in the broadest term as a cause, principle, or system of tenets and beliefs held to with ardour and faith by a group of people as more superior and true than others, it would include Hitler's Nazism under which six million Jews were massacred.
The point here is that nearly every religion stakes the claim that only it contains the only true message, which tends to create self-righteousness among believers to the exclusion of others. If the others feel the same way about their religious beliefs too, this is how factionalism and sectarianism arise to culminate in conflict — organised conflict of one group against the other, one community against the other and one country against the other especially where the religion is organised, and encompasses both public and private realm.
This is why it is absolutely imperative for the following to hold the following:
First, religion should be kept wherever possible a private observance, and state or public imposition of private morality and conduct should be kept minimum. Wherever possible, government, constitution and law should be secular to ensure neutrality for all religions so that they may flourish, side by side in tolerance and acceptance.
Secondly, discourse and fair comment should be allowed so that religious principles can interact with principles of humanism based on practical considerations of whatever is good and utilitarian for all humans, individually and collectively, irrespective of race and religious belief.
These are the only ways to combat narrow-mindedness, bigotry, factionalism and sectarianism — and terrorism. This is the only hope to combat violence and ensure peace and harmony.