The definition of the Oneness of Humanity is that all people are brothers, sisters, daughters, mothers, sons, and fathers of each other, and must have no enmity towards one another.

Bahaullah says:
Oh friends! Know that the curtain of unity has been raised, do not look at each other as strangers; you are all the fruits of one tree and the leaves of one branch.
Ref:Baha’u’llah, Majmu`iy-i alwah-i mubarak-ih, p. 265.

Abdul-Baha says:
The second principle of Bahaullah is the Oneness of Humanity. All of humanity are members of the human species. They are all servants of God. They have all been created by God. They are all divine children. God gives sustenance to all of them; nurtures all of them; is compassionate to all of them; why should we be uncompassionate? . . . The human species are all in the shadow of the Lord’s Grace. At most some are imperfect and must be perfected, [some] are ignorant and must be taught, [some] are sick and must be treated, [and some] are asleep and must be awakened. We should not be angry at an infant for being an infant. We should nurture him. We should not be angry at a sick person for not being healthy. We should have the greatest amount of mercy and kindness toward him. From these [examples] it is apparent that the enmity between religions should be completely wiped out; oppression and injustice should be removed, and instead, the greatest amount of kindness and affection should flow.
Ref:Abdu’l-Baha, Khatabat (Tehran), vol. 2, pp. 145-146.

According to Shoghi Effendi, the central axis of all Baha’i principles, and the ultimate goal of Baha’ism is the "Oneness of Humanity."
Ref:"The principle of Oneness of Humanity is the axis of all of Bahaullah’s teachings" and "The Oneness of Humanity is the main and ultimate goal of Baha’u’llah’s creed," Hushmand Fath A`zam, Nazm jahani Baha’i muntakhabati az athar sadiri az qalam hazrat wali amr Allah, 2nd ed. (Dundas [Canada]: n.p., 151 B. [1955]), pp. 49 and 57-58.

Abdul-Baha narrates from Bahaullah:
The Oneness of Humanity means that the entirety of humanity is subject to the blessings of [God] the Great and Glorious. They are the servants of one God and nurtured by his Highness, the Lord. Everyone is included in the mercy and the crown of humanity is the jewel of every head.
Ref:Abdu’l-Baha, Khatabat (Egypt), vol. 1, p. 31.

He emphasizes:
This means that Bahaullah declared the Oneness of Humanity so all members of humanity become brothers, sisters, daughters, mothers, sons, and fathers of each other.
Ref:Abdu’l-Baha, Khatabat (Egypt), vol. 1, p. 154.

Baha’is claim that all people are part of God’s creation. Skin color, beauty, and looks are no reason for the superiority of someone over another and must not be a cause of conflict and enmity. Instead, these differences are like the differences among the colors and types of flowers in a large field, which cause the field to flourish.
Ref: "‘Consider the flowers of a garden,’ `Abdu’l-Baha has written, ‘though differing in kind, color, form, and shape, yet, inasmuch as they are refreshed by the waters of one spring, revived by the breath of one wind, invigorated by the rays of one sun, this diversity increaseth their charm, and addeth unto their beauty. How unpleasing to the eye if all the flowers and plants, the leaves and blossoms, the fruits, the branches and the trees of that garden were all of the same shape and color! Diversity of hues, form and shape, enricheth and adorneth the garden, and heighteneth the effect thereof,’" Shoghi Effendi, The Advent of Divine Justice (US Baha’i Publishing Trust, 1990 [first pocket-size edition]), p. 54

In the presence of God, Turks, Persians, Whites, and Blacks are all the same, and none have any distinction over another.
Ref:"In the presence of God there is no [such thing] as English, French, Turk, or Persian. To God they are all the same," `Abd al-?amid Ishraq Khawari, Payam-i malakut, pp. 45-46 (citing `Abdu’l-Baha’); "‘God, `Abdu’l-Baha Himself declares, ‘maketh no distinction between the white and the black. If the hearts are pure both are acceptable unto Him. God is no respecter of persons on account of either color or race. All colors are acceptable unto Him, be they white, black, or yellow,’" Shoghi Effendi, The Advent of Divine Justice, p. 37

God is kind to all, so why should we not be kind to each other?
Ref: "All of humanity are members of the human species. They are all servants of God. They have all been created by God. They are all divine children. God gives sustenance to all of them; nurtures all of them; is compassionate to all of them; why should we be uncompassionate?" `Abdu’l-Baha, Khatabat (Tehran), vol. 2, pp. 145-146

The following questions should be kept in mind while analyzing this principle:

• What is the explicit meaning of the ‘Oneness of Humanity’? Does the Oneness of Humanity mean that people of all races and religions should become sisters, brothers, and friends or does it mean everyone should convert to Baha’ism?
• If the former definition applies and everyone-regardless of religion or creed-should live in peace and harmony, then why do Baha’i orders imply the contrary (as we will soon show)?
• If the meaning of the ‘Oneness of Humanity’ is that all people must become Baha’is, then who will be responsible for administrating the political matters of this unified Baha’i society, given that under Baha’i law, no Baha’i is allowed to interfere in political matters, either in words or in action?
  Ref:"Whoever interferes in political affairs is not a Baha’i," `Abd al-Hamid Ishraq Khawari, Ganjiniy-i Hudud wa ahkam, chap. 70, titled "Refraining From Interfering in Political Affairs," p. 336
• Do Baha’is have a particular plan for reaching the ‘Oneness of Humanity’ or is this merely a theoretical slogan?

• Oneness of Mankind: Introduction
• Is Oneness of Mankind a New Principle?
• Did the Founders of Bahai Faith Practice Oneness?
• Bahaullah's Attitute towards his Brothers and his Followers
• Bahai Treatment of Covenant Breakers
• Bahai Treatment of Thieves and Some More...
• Two Examples of Bahaullah's Treatment of Non-Bahais
• Is this Principle Correct from a Rational and Logical Perspective
• Contradictions and Conclusions

The articles have been based on the book "Avaze Dohol" - the Beating of the Drum by Masoud Basiti, Zahra Moradi.