Around the time of J. S. Mill, I think. The Industrial Revolution helped crystallize an elite political and academic movement which had the germs of scientific and quantitative thinking; but this movement has been far too busy fighting for its life each time it conflicts with religious mores, instead of being able to examine and improve itself. It should have developed far more productively by now if atheism had really caught on in Victorian England.

Anyway, I'm not as confident of the above as I am that we've passed the crossover point now. (Aside from the obvious political effects, the persistence of religion creates mental antibodies in atheists that make them extremely wary of anything reminiscent of some aspect of religion; this too is a source of bias that wouldn't exist were it not for religion's ubiquity.)