"Christo-" may be technically correct, as per the other answer, (even though I can't think of any words which use it), but I think in day-to-day speech a native speaker would be more likely to use "Christiano-" if they needed to invent a word on the spot.
For example (politics aside, talking only about usage): the word "Islamophobia" is gaining a lot of traction these days, and (very) occasionally you hear people talking about "Christianophobia" alongside it. Such as in the title of this book.
EDIT: I wrote this in a comment, I think it's worth adding to my main answer:
I think the real answer to the original question is "there isn't one", or "it depends". We're trying to look for the "correct" word based on obscure etymological rules, but most native speakers won't know those rules or care about them and there's very few contexts where we'd need to use a "combining form"