One is the number of unity, it is the first of numbers.
There is One God or Creator--- “The Sun of Divinity and of Reality has revealed itself in various mirrors. Though these mirrors are many, yet the Sun is one.” ‘Abdu’l-Bahá
There is One Word— “The bestowals of God are one; the reality of the divine religion is one. Consider how one and the same light has reflected itself in the different mirrors or manifestations of it”. ‘Abdu’l-Bahá
There is One human race/one human family— “All human creatures are the servants of God. All are submerged in the sea of His mercy. The Creator of all is one God; the Provider, the Giver, the Protector of all is one God. He is kind to all; why should we be unkind? All live beneath the shadow of His love; why should we hate each other?” ‘Abdu’l-Bahá
There is One homeland – “The whole earth is one homeland, all humanity is one people, and all are descendants of Adam. Religion is independent of politics. Religion promoteth praiseworthy qualities. The people of religion must strive for it to become a cause of fellowship amongst races; otherwise no benefit will result from it. Divine religions are the cause of fellowship, so that national prejudices may be abolished, and all peoples may become kind to one another and entirely severed from racial prejudice”. ‘Abdu’l-Bahá
“He has proclaimed the oneness of mankind and specialized religious teachings for existing human conditions. The first form of dissension arises from religious differences. Bahá’u’lláh has given full teachings to the world which are conducive to fellowship and unity in religion.” (‘Abdu’l-Bahá) In other words, “if the only creed of the Prophets is love and unity” (‘Abdu’l-Bahá), any other instructions were given to the people in the age they lived in, which differs from this age. They are therefore ephemeral and temporary.
Bahá’u’llá, the prophet and founder of the Baha’i Faith, explained the importance of recognizing our oneness as brothers and sisters in the human family. Nothing of lasting value can be achieved in the world otherwise. He wrote:“O Children of Men! Know ye not why we created you all from the same dust? That no one should exalt himself over the other. Ponder at all times in your hearts how you were created. Since we have created you all from one same substance it is incumbent on you to be even as one soul, to walk with the same feet, eat with the same mouth and dwell in the same land, that from your inmost being, by your deeds and actions, the signs of oneness and the essence of detachment may be made manifest.”
“The well-being of mankind, its peace and security are unattainable unless and until its unity is firmly established … Ye are the fruits of one tree, and the leaves of one branch. Deal ye one with another with the utmost love and harmony, with friendliness and fellowship. He Who is the Day Star of Truth beareth Me witness! So powerful is the light of unity that it can illuminate the whole earth.”
Religions are the expressions of Oneness, therefore harmony between the religions of today is a simple fact that needs expressing in all aspects of our lives.