O you who believe! do not take the Jews and the Christians as friends, they are friends of each other, and whoever among you turns to them (for friendship), then he is certainly of them, and Allah certainly does not guide unjust people.
Muslims are forbidden to take the Jews and the Christians as friends, an obvious reason is that they did not accept Muhammed (PBH) as the messenger of Allah (SWT) and the Quran as the message of Allah (SWT). Only those approach them for friendship in whose hearts is a disease and they are the losers.