“Christianity” isn’t a unifying philosophy; even more recently than 50 years ago, the most dedicated evangelical/fundamentalist Protestant Christians were campaigning against the single largest Christian denomination in the country (Catholicism) as vigorously as against non-Christians, and many of our key religious freedom rulings were a result of Christian or Christian-adjacent groups (Jehovah’s Witnesses figure particularly prominently here) resisting impositions by more dominant Christian groups. Kennedy got much the same kind of attacks for being an (actual) Catholic as were directed at Obama for being (supposedly) a Muslim.
And if the short history in America were not enough, look at the troubled history of Christian-Christian relations in Europe and the Near East.