Religion is an insidious thing. It is harmful in so many ways. It promotes wish-thinking and superstition, it retards scientific progress and in turn it holds back the potential of our society at large. It is oppressive to minorities and it suppresses unorthodox thinking, while at the same time placing an exaggerated value on suffering and on resisting our true nature.
But greater than any of that, the institution of religion has done most of it’s harm by way of systematic and often violent oppression of women. When you discount the value of half of your population, as religion does universally, you effectively cut the legs off of any chance your civilization has of rising beyond mere subsistence.