Allah has made the relationship of the Muslim to a Muslim more sacred than any relationship, even that of blood, culture or nation. He has united us into one ummah, the Muslim ummah, and has made us responsible for each other. The relationship between the Muslims is a sacred one:
...A Muslim is the brother of a Muslim. He neither oppresses him nor humiliates him nor looks down upon him. The piety is here, (and while saying so) he pointed towards his chest thrice. It is a serious evil for a Muslim that he should look down upon his brother Muslim. All things of a Muslim are inviolable for his brother in faith; his blood, his wealth and his honor. Sahih Muslim Book 31, Number 6219.