Christianity is not a nationality, a person who is not baptised is not Christian. There are many cases of atheists, even of Jewish converts who converted abroad or non-orthodox and Jews who fail to proove their Judaism after Aliyah, who are registered as Christians. A friend of mine protested against it but was told he is Christian because he is not Jewish and was born in New Zealand.
Ya'alon okays administrative detention of Jewish extremist suspected in church arson (Haaretz)
from the article: Israeli author Yoram Kaniuk asks court to cancel his 'Jewish' status