The Quran

إِنَّمَا المُؤمِنونَ إِخوَةٌ فَأَصلِحوا بَينَ أَخَوَيكُم ۚ وَاتَّقُوا اللَّهَ لَعَلَّكُم تُرحَمونَ

Ali Unal

The believers are but brothers, so make peace between your brothers; and keep from disobedience to God in reverence for Him and piety (particularly in your duties toward one another as brothers), so that you may be shown mercy (granted a good, virtuous life in the world as individuals and as a community, and eternal happiness in the Hereafter).¹

The last two verses mean that all the believers are brothers and sisters, who are dutiful to one another in this relationship. It hints that there may be quarrels, even fighting, among brothers, which may sometimes arise from rivalry and jealousy. Even if they quarrel and fight with one another, they are still brothers and sisters, and brotherhood and sisterhood require peace. If, despite the fact that there cannot be enmity among them, two parties of believers dispute or fight with each other, the other believers, who are brothers and sisters to them, must reconcile them immediately and make peace between them as required by such a relationship. When they are reconciled and have made peace, they must be meticulous in acting according to the precepts of justice. Since quarrels among brothers and sisters usually break out because of rivalry and jealousy, dispensing justice with great care is particularly important.
Brotherhood (and sisterhood) is very important for both the individual and social life of the believers. In particular, their prosperity in the world and superiority against their enemies depend on faith and this relationship. If they clash with one another and divide into rival groups, it is inevitable that they will weaken and be defeated by their enemies. For this reason, both the Qur’ān and God’s Messenger, upon him be peace and blessings, have greatly stressed the importance of brotherhood and sisterhood. God’s Messenger, upon him be peace and blessings, used to demand the allegiance of the believers on the conditions that they had to perform the Prescribed Prayers, paying the Prescribed Purifying Alms, and be well-wishers of the believers. He also said: “Cursing a Muslim is a transgression, and fighting with him amounts to unbelief” (al-Bukhārī, “Īmān,” 36). Again, he said: “A Muslim is a brother of another Muslim. He never wrongs him nor makes him devoid of his support. There is no greater offense for a Muslim than despising his Muslim brother” (al-Bukhārī, “Adab,” 57–58; Muslim, “Birr,” 28–34). He also said: “Believers are like a single body in loving and showing mercy to one another; (just as the whole body suffers from any suffering in any part of the body,) so too, will all believers suffer because of the suffering of a believer” (al-Bukhārī, “Adab,” 122; Muslim, “Birr,” 66). (For the importance of brotherhood and how it can be realized and preserved, see Said Nursi, Lem’alar, “20. Lem’a.”)