You can find the answer to this in Chapter 14. There, Diamond tells us that religion is something of a mixed bag for the average person in society.
On the good side, religion gives people a way of living together without killing one another. Large societies are made up of people who are unrelated and may not even know one another. They need a reason to avoid conflict and violence. Religion is such a reason.
On the bad side, religion is used to push people to do things that are not in their interests. It is used to justify obeying political leaders and giving them money. It is also used to give people a reason to sacrifice their lives in battle for their society.
In these ways, religion can be good in a way, but bad in other ways for the average person.