John Gray and A. C. Grayling go toe-to-toe at the National Interest's site. Gray wrote the initial, longer attack piece. Both authors fight dirty. That is, ad hominems all over the place! Indeed, Grayling's rebuttal consists of little else -- I hope he posts a longer rebuttal than the quick one he's offered.
I'm more sympathetic to Gray's position. Grayling, Hitchens and the others take a too simplistic view of religion's historical influence. One example is their neglect of abolitionism's roots, especially in the UK. Even those British philosophers, like Hume, who opposed slavery, tended still to be racist. Their opponents were British writers who appealed to the Biblical notion that we're all equal in the eyes of God.
Gray notes the positive influence of some English philosophers (such as Locke) in promoting toleration and respect for freedom of expression, but he doesn't mention Milton. Is that a deliberate omission? Regardless, while Milton the Puritan likely wouldn't measure up to our current ideals of toleration, he played an important role in their development.
Gray points to the malign influence of 20th-Century atheistic regimes, noting that so far, atheists in power have been illiberal. His approach reminds me of that of Marilynn Robinson. Both authors round up the usual benighted, wannabe rationalists (usually Marxists), document their crimes, and then add: 'Sure, most rationalists will deny that these characters are what they had in mind when talking about reason, but that's the same move that we religious folks make when we disavow the Crusaders. See? How do you like it?'
Some of Gray's points on this score reminded me of Paul Tillich's application of the notion of idolatry to recent history. Tillich itemized the terrors that follow when one's ultimate concern is invested not in a forever transcendent, divine target but, rather, in some finite, wholly immanent object (such as a human leader or the state).
Here's where a point can be made in support of Grayling's side of the debate. Granted, the atheistic rulers of the past century left a sad legacy, but that is because they extended their devotion to a concrete, historical object (such as the state). By contrast, Grayling's more liberal brand of atheism takes as its focus the ideal of reason. Given our inevitable failure to be perfectly rational, to do so is to direct one's trust towards an ever-receding, transcendent ideal (rather than towards any historical figure).
So, many religious and atheistic ideologies of the past were equally dangerous because their trust, loyalty or ultimate concern resided in a concrete, contemporary figure in the government or church instead of in a never wholly manifested ideal (secular reason or an infinite God) that stands forever in judgment of our historical, fallible efforts.
Gray would likely acknowledge something like this point but reiterate that most ideologies, whether religious or atheistic, have an equally sorry track record when put into practice.