Is this principle something new? Had it not been heard before the Baha’is declared it?

If this principle means that God is the creator of all humans, that Adam and Eve are the father and mother of all people, and that God wishes that all persons be inclined towards goodness, then this is not a new belief nor a novel teaching.

For example these two verses from the Quran invite all people to oneness and show that color, ethnicity, and gender are not means of superiority:

O mankind, indeed We have created you from male and female and made you peoples and tribes that you may know one another. Indeed, the most noble of you in the sight of Allah is the most righteous of you. Indeed, Allah is Knowing and Acquainted.
(Ref: Quran, 49:13)

And hold firmly to the rope of Allah all together and do not become divided.
(Ref: Quran, 3:103)

When most Iranians hear the phrase ‘The Oneness of Humanity,’ they are reminded of Sa`di’s famous poem: Human beings are members of a whole
In creation of one essence and soul

If one member is afflicted with pain
Other members uneasy will remain.

This poem was composed in the 13th century, hundreds of years before Bahaullah was born. So it seems this Baha’i principle is nothing novel or new. In fact, Abdul Baha confesses that this teaching existed in all Divine Religions:

The foundation laid by all Prophets, is the foundation of Bahaullah and that foundation is the Oneness of Humanity. That foundation is general compassion; that foundation is universal peace between governments.
Abdul Baha, Khatabat (Tehran), vol. 2, p. 286

All Divine Prophets struggled for the Oneness of Humanity and served humanity, for the foundation of the divine teachings is the Oneness of Humanity. Moses served the Oneness of Humanity, Jesus established the Oneness of Humanity, Mohammad declared the Oneness of Humanity. The Bible, Torah, and Quran established the foundation of the Oneness of Humanity. The law of God is one and the religion of God is one and that is affection and kindness. His Highness, Bahaullah, renewed the teachings of the prophets.
Abdul Baha, Khatabat (Egypt), vol. 1, pp. 18–19

Consider history. What has brought unity to nations, morality to peoples and benefits to mankind? If we reflect upon it, we will find that establishing the divine religions has been the greatest means toward accomplishing the oneness of humanity . . . The divine religions are collective centers in which diverse standpoints may meet, agree and unify. They accomplish oneness of native lands, races and policies. For instance, Christ united various nations, brought peace to warring peoples and established the oneness of humankind.
Abdul Baha, The Promulgation of Universal Peace, p. 158

Abdul Baha clearly announces with great emphasis that Moses, Jesus, and Muhammad all invited people to the Oneness of Humanity. He is clearly confessing that this teaching is not new. Despite this, at other times he uttered the contrary and claimed that the previous Prophets divided humanity and it was only Bahaullah that proclaimed the Oneness of Humanity:

In all religious teachings of the past the human world has been represented as divided into two parts: one known as the people of the Book of God, or the pure tree, and the other the people of infidelity and error, or the evil tree. The former were considered as belonging to the faithful, and the others to the hosts of the irreligious and infidel - one part of humanity the recipients of divine mercy, and the other the object of the wrath of their Creator. Bahaullah removed this by proclaiming the oneness of the world of humanity, and this principle is specialized in His teachings, for He has submerged all mankind in the sea of divine generosity.
Abdul Baha, The Promulgation of Universal Peace, p. 454

Unfortunately, as will be demonstrated in the next section, Abdul Baha continuously changed his words depending on the audience he was preaching to.

• 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.