To say that religious people have more self-control than people who are not religious based on comparative studies means that McCullough would have had to find identifiable groups of atheists and agnostics to compare to the groups of religious people.

One thing about atheists and agnostics: we have not been organized into groups. Until lately, we haven't had organizations at all, because we found it unsafe to self-identify.

He may have found psychological techniques that religion has developed or capitalized on. If so, then these psychological techniques that can be used by anyone with a forebrain, not merely by people who are religious.