I'm thinking of writing this (as you can tell, from my last few posts on this blog); although I'm did wonder whether 'An Atheist's Defence of Christianity' mightn't be a better title.
One key thing is to make clear that this isn't an exercise in a mode of lost-faith nostalgia, or sepia-tinted affection for the charming architecture and rites of (for instance) the Church of England. As it happens, I am neither nostalgic nor especially affectionate for that institution. My mother's father was a Vicar, but my mother was herself an atheist from an early age, and my father lost his evangelical faith in his teens. I was raised, non-dogmatically but effectively, in disbelief, which leaves me with nothing (personally speaking) about which to be nostalgic, and no especial emotional connection with the community of Anglicans.
So what would the point be? It would in part be about what has become a pretty polarised and aggressive 'pro-faith' vs 'Dawkinsian anti-faith' debate. The heat, here, is of a similar temperature to (although more civilised mores, or more social inertia, has resulted in a lower body count than) the old debates on doctrine, in which two groups who shared not only religious faith but belief in the same God and saviour, the same Holy Book and many other points of identity, nevertheless made war upon one another in the name of suppressing heresy and slaughtered by the thousand. I have a heresy of my own: it is that atheism is the only non-blasphemous Christianity. I'll need more than a blog post to argue it, though.
Of course, a Christian might say: the last person qualified to "apologise" for Christianity is an atheist! You're further removed from the core of what Christianity entails -- belief in God, a soul, the divinity of Christ, the incarnation, substitutionary atonement and so on. Or else a Christian might say, in less hostile mode: but why write An Atheist's Apology for Christianity? Why not just ... become a Christian! Both objections touch on a key point.
In a nutshell, the argument would be that Christianity is radically concerned with the excluded, the un-chosen people, the scums and bums; and that once it becomes the dominant religion on the planet, the key category of excluded becomes 'the unbelievers'. So only an atheist can truly be a Christian. In appropriately paradoxical mode. This may come over merely as glib; but I hope not. It is (or it seems to me) an observation both profound and important.
On balance I think 'An Atheist's Defence of Christianity' isn't as good a title as 'An Atheist's Apology for Christianity'. 'Apologia' in Latin means 'defence' or 'apology' but 'apologo' means 'reject, spurn'.