What horrors can be justified by appeal to the authority of the non-rational, the traditional, the superstitious, the suppositious, the evidentially unsupported, and so forth, history too often bloodily teaches.John Gray seems to hold the view, to my mind equally wrong, that religion is essential to morality and peace. He thinks a review of communism, fascism and other twentieth-century evils demonstrates the immorality of atheism and the absurdity of faith in reason. He and Grayling are mirror images of each other, neither able to see that religion is neither the source of the human tragedy nor its cure. On average religion does not, so far as I can tell, make people or historical epochs either better or worse. Both atheists and crusaders commit atrocities, and both are capable of great humanity.
Religion is not some outside force that acts on humanity; it is part of humanity. To be religious does not change what people are, because it is simply an expression of what they are. Religion contains great beauty because creating beauty is part of what humans do; it includes oppression and violence because oppression and violence are part of what humans do. And because oppression and violence are integral parts of humanity, rejecting religion will not make them go away.