It’s certainly true that violence predates religion. It’s also certainly true that violence can and does happen today without religion, and that getting rid of religion would not get rid of all violence.

However, religion is clearly an aggravating factor which amplifies and enables violence. There are clear reasons for this in the very basis of religions. Because religions rely on private revelation (scripture) as a source of truth, those who disagree cannot “check the source”, to see if the private revelation is “real” as opposed to just something someone made up. Since private revelation cannot be objectively tested, the only thing left is to resolve the dispute by fighting, which is what has been proven time and again over the past several thousand years.

Without religion, people have to rely on objectively testable data for their worldview. If there is a disagreement, simply repeat the test, consult those who did, etc. No need to fight. If one wants to keep an idea of a pantheistic god, then that data could be called “public revelation”.

That’s one, perhaps the biggest, reason why religion is violent, why reducing religion reduces violence (shown many times by studies), and why supernatural religion needs to be abandoned if we are to have a just, healthy and peaceful world. It’s a lot like asking if smoking causes cancer – the answer is no – mutations cause cancer. But those mutations are encouraged and made much more likely if one smokes.

There are other reasons – like clear verses in most Bibles, the Qu’ran, and so on, which encourage violence. Those might be avoided with different scripture, but the supernatural basis of most religions makes the first reason unavoidable, regardless of how many counterexamples this book might list.