And never will the Jews or the Christians approve of you until you follow their millah (religious community, religion, creed, faith). Say, "Indeed, the guidance of Allah is the [only] guidance." If you were to follow their desires after what has come to you of knowledge, you would have against Allah no protector or helper.
[Qur’aan – 2:120]
Imaam al-Shaafi'i, Ahmad bin Hanbal, Daawud, Abu Hanifah, and a group of scholars have said that all of the kaafirs are one millah (nation) based on this Verse. In the Verse, Allaah uses the words "their millah" to refer to all of them. They have also used the following hadith to support this view of theirs:
It was narrated from ‘Amr bin Shu’aib, from his father, from his grandfather, that the Messenger of Allah (sallallaahu alayhi wa sallam) said:
“People of two different millah do not inherit from one another.”
[Sunan Ibn Maajah Hadith No. 2731]
The two different millah here, according to them, refers to Islaam and Kufr.