They are of two kinds: either your brother in faith or your equal in creation.
فَإِنَّهُمْ صِنْفَانِ: إِمَّا أَخٌ لَكَ فِي الدِّينِ وَإِمَّا نَظِيرٌ لَكَ فِي الْخَلْقِ
Fe-innehüm sınfân: immâ ehun leke fi'd-dîn ve immâ nazîrun leke fi'l-halk.
Ali ibn Abi Talib, Nahj al-Balagha, Letter 53 (the covenant to Malik al-Ashtar).
This sentence comes from a letter.
Written to Malik al-Ashtar, appointed governor of Egypt.
To someone about to hold power.
Whoever holds power begins to see those before him as a crowd.
The letter reminds him:
some of them are your brothers in faith,
some your equals in creation.
There is no third kind.
No door is left for "stranger".
No one is left outside mercy.
The higher the office,
the easier it is to forget these two words.