The greatest threat to Christianity is ...
misguided Christians, just as the greatest threat to Islam is misguided Muslims and the greatest threat to Judaism is misguided Jews. Religious insiders can do harm to their religion in ways that outsiders never could. This is especially true in a pluralistic world, where religions are credible to the degree they bring benefits to outsiders.
My friend Rabbi Michael Lerner explains why, in poignant reflections on the current situation in Israel-Palestine, here: