The position is that religion causes war. Just like, say, alcohol causes inebriation and sunlight makes things grow.
If we wanted to test the alcohol/inebriation or sun/growth hypotheses scientifically, what would we do? Quite simple: Remove the alcohol from whatever drinks we are serving and see if our clients are still inebriated. Same with the sun/growth theory: Remove the sunlight and see if things still grow.
With the religion/war hypothesis, we don't have to actually make a clinical study--it's already been done for us. In the 20th century, we saw the most disastrous wars of history, both in Europe and in the Far East. Tell me, which of these were centered around religious disputes?