Is it permissible for a Muslim to have Christian friends and is there any limit to this friendship?
It is not permissible to take the infidels as friends or companions. However, they should be treated kindly as instructed by our Shari`ah (Islamic Law). We should treat them with the best saying and argue with them in the best way because of the saying of Allah (Exalted be He),
And speak good to people [i.e. Enjoin righteousness and forbid evil, and say the truth about Muhammad صلى الله عليه وسلم]
(Surah Al-Baqarah), His saying,
And by the Mercy of Allâh, you dealt with them gently. And had you been severe and harsh-hearted, they would have broken away from about you
And argue not with the people of the Scripture (Jews and Christians), unless it be in (a way) that is better (with good words and in good manner, inviting them to Islâmic Monotheism with His Verses), except with such of them as do wrong