The original publication has been removed and there is no online copy I can point to. It was called "Survival of the Godliest" by the demographer Phillip Longman. I did find what looks like a partial or complete extract here
The thesis was that a number of behaviors of religious adherents conferred survival benefits. Specifically, that religious people have more children than non-religious people. The result is more religious people and less non-religious people over time. And the more religious people are, the greater number of children they have, therefore the religious community becomes more fundamentalist over time.