Never mind the actual post, The ‘Atheistic’ Character of Christianity and the Question of Christ; the meat of Alastair Roberts' point is in his reply to my comment. (You'll have to scroll down a bit; I don't see any way of linking to individual comments on his blog.)
Roberts seems to make two fundamental points. First, popular forms of Christianity are "heterodox" and "bizarre". His "bog-standard orthodox Christian tradition" is the real Christianity; to address other forms is to attack a straw man. Despite Roberts' effort to find common ground with atheism, Roberts seems to consider the fundamentalists so marginal that they are less of a problem than atheists themselves. Second, atheists seem to have forgotten that Christian ideas, the ideas of his sort of Christianity, are "part of the DNA of Western culture." By abandoning Christianity, atheists are losing a critical grounding for Western culture, without offering a satisfactory alternative. But Roberts is, I think, fundamentally missing the point of the modern atheist movement, which is primarily a struggle against fundamentalism.
Roberts is aware of a wide range of atheist beliefs. He praises (or does he damn it with faint praise?) a strain of atheist thought that is relentlessly questioning and critical. But the modern atheist movement — the New Atheists — aren't really part of the deeper philosophical struggle against theism. Our primary targets are the fundamentalists, those who would use a view of God — a view that I suspect Roberts would find "heterodox" — to assert supernatural status to their petty hang-ups and small-minded prejudices. If Roberts' brand of Christianity does not share those hang-ups and prejudices, good for him; if the shoe doesn't fit, he is not obliged to wear it. If Roberts does not want to struggle against the fundamentalists, that's his choice, but we do want to struggle against the fundamentalists. When New Atheists attack the philosophical underpinnings of fundamentalism, we are not (at least not necessarily) attacking the underpinnings of Roberts own theology. That the New Atheists often seem unaware of his theology is primarily because his theology is not directly relevant to our struggle.
But of course, the deeper philosophical critique against theism, which definitely does include Roberts' theology, is also important. But I think atheists expect a higher burden from Roberts than he would prefer. Roberts claims that Christianity is historically important, but atheists quickly grant that claim. From an historical argument, one can demand only that we include a considerable body of Christian thought in the canon of the humanities, and we do include that body of thought. (Whether we adequately promote that thought as something every educated person should be familiar with is a different argument, and atheists as a group do not have much standing to set the academic curriculum.) Yes, Christian thinkers, operating in various theological contexts, have made important contributions to Western values. Thanks, but what have you done for us lately? But Roberts is not making only an historical argument.
Roberts appears to believe that not only is Christianity an important historical force, but that it is a philosophically essential component of the edifice of Western cultural values, values that atheists themselves endorse. To deny Christianity, Roberts' asserts, is to deny the philosophical underpinnings of our notions of rights, even our notions of skepticism and critical investigation. To assert those same rights and methodologies without Christianity — the right sort of Christianity, of course — is to work on borrowed ontological capital. Hopefully, he will develop this argument further.
But this argument does not require that atheists have a deep understanding of any particular theology; atheists need only offer our own satisfactory grounding, without any gods or supernatural forces. (Of course, I doubt that we will ever satisfy Roberts himself, so we need only satisfy ourselves.) But this grounding is easy to provide. We can observe, and draw scientific conclusions from our observations, many characteristics and properties of human beings in general. We are beings that want to be happy, and we are social beings who have evolved and learned to be concerned with the happiness of others. We can also observe that trying to manage the happiness of large numbers of people is an extremely complicated task, too complex to optimize analytically. We can view all of our discourse on rights, freedoms, ethics, and justice as simply that: a dialectical conversation to address the burning question of how we can all be as happy as possible. We don't need any god to want to be happy, and we don't need any god to observe that we have evolved as a social species, concerned with the happiness of others.
We're all very pleased that Roberts et al. have come to many of the same conclusions that secular humanists have come to, even if they express those conclusions in a theological rather than a naturalistic narrative. But we do not need to be deeply familiar with that theological narrative to address either of our goals. We do not need to address the humanistic theological account to address nonhumanistic religion. To the extent that we struggle against fundamentalism, we don't need to talk about non-fundamentalist theology. And we don't need to address any kind of theology to find that our Western values — the good ones, at least — can very easily rest on our biological and social natures.