As an atheist, I am perfectly happy to identify as "Christians" all those who claim that label for themselves, be they Catholics, Pentecostals, Mormons, unaffiliated home-churchers, Lutherans, Eastern Orothodox, Jehovah's Witnesses, Baptists, Christian Spiritualists, Quakers, Christian Scientists, Congregationalists, etc. etc. I include Unitarian Universalists who specifically identify as Christians, while acknowledging that not all UU congregants do.
Who am I to judge what constitutes a "true" Christian? Furthermore, as McEwan states much more eloquently than I,
Christianity has a 2,000-year history that has seen countless iterations of the religion based on countless interpretation of the text and shaped to fit countless times and spaces and needs in disparate cultures all around the world. Christians have done great things, and not-so-great things—and anyone who makes the personal choice to carry the Christian mantle associates themselves with a history that includes all the good stuff and all the shitty stuff, too. One can't say, "I only associate with the good Christianity—not the inquisitions and the genocides and the warmongering and the colonialism and the institutional misogyny, homophobia, transphobia, racism, anti-Semitism…"That's all part of Christianity's legacy, too—and it just isn't intellectually honest to say, "Well, those weren't real Christians." Yes, they were. And so are the Christians who do shitty stuff today.