"It's pretty hard to contest the argument that religious belief has caused far, far more harm than good in this world."

Nah, pretty easy to contest that argument.
First what are you comparing it to? There is a lot people who fall in the category of having religious belief. For instance you could say five fingered humans have done far more harm than compared to six fingered humans- sheer numbers and duration of time of people holding religious beliefs will give you a distorted view.
Ask yourselves what nation has done good and is comprised of people who don't hold religious beliefs. Or even what nation even has high percentage of people who don't have religious beliefs and mostly done good.
So perhaps you believe that only in future will nations get to point of having higher percentage of people who are not religious. What about mere groups both small and large that comprised of people who don't have religious beliefs- can you identify them and give any examples of the good that they have done? Or if that too hard, how about some individuals?

Can you even explain what you mean by good and harm and how you suggest they be compared in an objective manner?