why the christians were called nasara
[مَنْ أَنصَارِى إِلَى اللَّهِ قَالَ الْحَوَارِيُّونَ نَحْنُ أَنْصَارُ اللَّهِ]
("Who will be my helpers in Allah's cause'' Al-Hawariyyun said: "We are the helpers of Allah.'') (61:14)
It was said that they were called `Nasara', because they inhabited a land called An-Nasirah (Nazareth), as Qatadah, Ibn Jurayj and Ibn `Abbas were reported to have said, Allah knows best. Nasara is certainly plural for Nasran.
When Allah sent Muhammad as the Last and Final Prophet and Messenger to all of the Children of Adam, mankind was required to believe in him, obey him and refrain from what he prohibited them; those who do this are true believers. The Ummah of Muhammad was called `Mu'minin' (believers), because of the depth of their faith and certainty, and because they believe in all of the previous Prophets and matters of the Unseen.