The utterance of God is a lamp, whose light is these words: 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.
…Consort with all men, O people of Bahá, in a spirit of friendliness and fellowship. If ye be aware of a certain truth, if ye possess a jewel, of which others are deprived, share it with them in a language of utmost kindliness and good-will. If it be accepted, if it fulfil its purpose, your object is attained. If any one should refuse it, leave him unto himself, and beseech God to guide him. Beware lest ye deal unkindly with him. A kindly tongue is the lodestone of the hearts of men. It is the bread of the spirit, it clotheth the words with meaning, it is the fountain of the light of wisdom and understanding…”
Gleanings From the Writings of Bahá’u’lláh, pages 288 & 289
“A fundamental teaching of Bahá’u’lláh is the oneness of the world of humanity. Addressing mankind, He says, “Ye are all leaves of one tree and the fruits of one branch.” By this it is meant that the world of humanity is like a tree, the nations or peoples are the different limbs or branches of that tree, and the individual human creatures are as the fruits and blossoms thereof. In this way Bahá’u’lláh expressed the oneness of humankind, whereas 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. Bahá’u’lláh 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. Some are asleep; they need to be awakened. Some are ailing; they need to be healed. Some are immature as children; they need to be trained. But all are recipients of the bounty and bestowals of God”
The Promulgation of Universal Peace, page 455