Religion

A religion is a set of aesthetic practices and preferences.

Or maybe an ethical code.

A collections of habits & rituals.

A set of truth claims.

A way of life; a recipe for living.

A literary & intellectual tradition.

A proxy for group identification or social bonding.

etc...  

So the question of atheism vs. theism comes down to one thing that religion is: a set of truth claims. The  atheist and the theist are in complete agreement here, that this is at the heart of the question.  Theism in this sense only arises because of the possibility of a-theism. I remember reading Descartes and thinking: who are these atheists he is talking about?  Surely nobody ever even considered that...

The troubles between Catholics and Protestants in Northern Ireland certainly weren't about truth claims, which theology was right. It wasn't about which ethical code to follow, since to the extent that Protestants and Catholics differ, they don't differ about whether you can kill people.  Were they fighting about which rituals to observe.  No.  It seems that religion here was only proxy for two ethnic groups, one natively Irish and one essentially British but living in Northern Ireland.

If it were a religious conflict, then you could have put a Bishop and a Protestant leader together in a room and resolved it easily.  Each leader would just tell their congregants to stop fighting.

If you challenge a religious person on truth claims, they will often retreat into aesthetics or some of the other things religion might be.