The Qur'an says, "O Men, have fear of your Lord, who created you from a single soul. From that soul He created its mate, and through them, He scattered the earth with countless men and women." (4:1)
This shows that men and women have been created from the same substance. Their being physically one, of necessity, demands their spiritual oneness.
Making a similar point, the Prophet Muhammad said that all human beings are brothers (Abu Dawud). This gives rise to the concept of a common brotherhood, and without doubt, it is this sense of brotherhood which generates the strongest feeling of oneness and togetherness among different people.
That is to say that when all human beings in this world are virtually blood brothers, they must, as this concept necessarily demands, live as brothers in spirit too. Any other way of living is a deviation from the reality.
I should like to refer here to an incident which illustrates this point. In 1893, Swami Vivekananda went to Chicago to participate in the Parliament of Religions. As the Encyclopaedia Britannica puts it, his was a 'sensational appearance.' (15/623) On that occasion all the speakers at the conference followed the common practice of addressing the audience as "Ladies and gentlemen." But when Swami Vivekananda took the stage, he addressed his hearers as "Sisters and brothers of America." No sooner were the words out of his mouth than the hall resounded with a long burst of applause. Of all the delegates at the conference, Swamiji received the greatest ovation.
The reason for this was that the form of address, "Ladies and gentlemen" produces a sense of alienation and strangeness, whereas the phrase "Sisters and Brothers" introduces a note of closeness and familiarity. By using this phrase, Swami Vivekananda touched a cord in the hearts of those of different creeds and colours. Their natural feeling of unity was awakened, and then what ensued fulfilled the best of expectations. All of a sudden, the gaps between them were bridged. They all began to feel themselves what they really were, and for that moment, physical divisions disappeared and were replaced by a rare spiritual unity.