Islam had appeared in a tribal society wherein justice as value was almost unknown. Tribal society has certain oral conventions and has no written law and oral conventions develop over a period of time and need for justice, as such, is hardly felt. One more reason for this is private property and wealth does not exist in such society. The only persistent law is that of revenge and retaliation. The Qur'an refers to it as a law of retaliation. However, situation in Mecca was much more complex as Meccan tribals had taken to international trade and Meccan society was in transition from tribal to a mercantile society and the institution of property and wealth had already developed and many disputes were arising between traders from time to time and there was no legal frame-work to resolve them. One such dispute before Islam was referred to the Prophet (PBUH) and the Prophet solved it using justice as a rule and it was on this occasion that he is reported to have said that he would not accept 100 red camels in lieu of that.
This dispute involved a Yemeni trader who was cheated by some Meccan traders. This incident shows that trade and property disputes had begun to take place in Meccan society which had developed into a centre of international trade and finance. Thus the Qur'an made justice the central value and made it a part of Islamic morality. As it was not easy in that society to get concept of justice accepted, Qur'an, in order to emphasise its importance made it Allah's name. Allah is called 'Adil i.e. Just. Thus if you worship Allah you must be doer of justice.
Qur'an also made it an integral part of Islamic morality and declared that "Be just, it is nearest to piety" (5:8). Now piety is a moral concept and the word for this is, in Qur'anic language taqwa. Actually the word taqwa means to refrain i.e. to refrain from evil and thus one who is muttaqi i.e. observant of taqwa refrains from all that is evil and being unjust is evil. So a Muslim who is muttaqi refrains from injustice. Thus justice became part of Islamic morality. Unfortunately in later days when rulers became highly unjust and oppressors of their opponents, taqwa lost its original meaning and it came to be associated with ritual piety and a muttaqi Muslim was one who prayed five times and fasted during the month of Ramzan and performed all Islamic 'ibadat devoid of social morality. But the Qur'an clearly pronounces that taqwa is much more than ritual observations and is integrally connected with justice. Thus a pious Muslim has to be just and his moral integrity should also be beyond any ken of doubt. Thus Qur'an says, "O you who believe, be maintainers for justice, bearers of witness for Allah, even though it be against your own selves or (your) parents or near relatives - whether he be rich or poor, Allah has a better right over them both." (4:135) Thus the words that "Allah has better rights over them" again shows that justice is part of Islamic morality and there cannot be getting away from it, if one wants to be a pious Muslim who keeps his duty to Allah. Also in a tribal society there was no concept of rule of law and emphasising role of justice is highly necessary for the rule of law. Thus in this respect also Qur'an says," And if you judge, judge between them with equity. Surely Allah loves those who are just." (5:42) Thus in the Arab society the rule of law began with the revelation of Qur'an. Since international trade was developing fast and was expanding, tribal customs and traditions could not have sufficed and it was becoming a social need to establish certain legal concepts beyond and transcending tribal limits. A universal society cannot be bound by narrow tribal limits and Islam was nothing if not universal. Thus Islam adopted universal standards of law and morality and concept of justice is very important for such a morality. Islam had to spread beyond the limits of Arabia and it did within few years of its origin and thus tribal morality of hardly any use in such a case. Islam was anything but tribal religion. There were far more advanced civilisations than Arab tribal culture. All its laws and standards of morality had to be universal in nature. Islam spread across civilisations and appealed to people as far as India and China, countries, which had far more richer and complex civilizational institutions. To begin with all Arabs adopted this religion as for them it became national religion. But even for Arabs it had to appeal to all their tribes as tribal norms, customs and traditions varied from tribe to tribe. These tribes had fought each other for decades and had maintained their separate identities for centuries. Even to unite all these tribes was a grave challenge. To meet these challenges Islam had to appeal to all tribes. If it had adopted norms of only Quraysh, a leading tribe in which Islam first appeared, other tribes would have rejected it outright. Islam transcended all narrow limits and easily united all tribes through its universal standards of morality. Quraysh was most dominant tribe among Arabs and Qurayshites looked down upon other tribes. Islam, though it appeared among them, gave no place of distinction to them and once a person entered in Islam, had no superiority over others even if they happened to be from Quraysh.
There lies the reason why Islam spread so fast among weaker sections of society. Islam became a powerful magnate for all weaker sections of the world. Among the principles of social justice and morality was the principle of equality and equity. Qur'an states that all are equal in the eyes of Allah and one closest of Allah is one who is most pious i.e. most just. The universality of Islam could be judged from very revolutionary declaration of Qur'an, "O humankind, surely We have created you from a male and a female and made you tribes and families that you may know each other. Surely; the noblest of you with Allah is the most pious of you. Surely Allah is knowing, Aware." (49:13) (emphasis added).
Thus it will be seen that tribes and families are not marks of distinction over the other but only for the purpose of knowing or identifying each other. Otherwise all have been borne of male and female and there is no question of superiority of one tribe or family over the other and only mark of distinction could be taqwa i.e. being highly moral person observing high norms of justice and only such persons are closest to Allah.