A correspondent has come across a book written by an American Christian priest who has used a small 'c' for Christian. Finding this 'odd, if not slightly disrespectful', she asks: 'Is there a rule for writing Christian or is it personal choice?'
The dictionaries are generally scrupulous about matters of capitalization, making such distinctions as 'cap'(italized), 'sometimes cap', 'usually cap', and so on. I've just looked at half a dozen: the OED, Longman, Chambers, and Collins all give it with a capital, and make no qualification. There is a hint of variation in US dictionaries: Random House gives it only with a capital, but Webster (which lists all its headwords in lower-case) says 'usu cap'.
None of these dictionaries distinguishes between the adjective and noun use: to my intuition, I would find a christian person more acceptable than a christian. With some people, intuitions are probably being affected by the Internet, where a general trend towards lower-case usage is noticeable.
But the norm is clear: Christian. And as publisher copy-editors tend to follow dictionary practice, what is surprising is to find a lower-case version allowed through in a book. This suggests that the author is making some sort of point. If so, I would expect to see it reflected in other, related usages, such as bible for Bible, or protestant for Protestant. However, my correspondent says that the author writes Episcopal priest, with a capital. So it does rather look as if some point is being made, consciously or unconsciously. There are many possible interpretations. For example, I can imagine a Dawkinsian writer lower-casing certain religious names to make a cheap point; I can imagine a denominational writer lower-casing all names bar one to make a point about identity; and I can imagine a mystical writer lower-casing names to make a point about humility.
It would make an interesting stylistic exercise to work out what was going on in this case. Or, of course, one could just ask the author.