The oneness of religion is a core principle of Bahá’u’lláh’s teachings. The following is an incomplete introduction.
The principle is perhaps also one of the most widely misunderstood of Bahá’u’lláh’s principles. In one form, the misunderstanding is that this principles involves in some sense ‘combining’ different religions or taking elements from different religions to create a new religion.
This is not at all the meaning of oneness of religion.
It is easier perhaps to understand the concept if we think of the term “religion” in the phrase ‘oneness of religion’ as not referring so much to individual religions (such as Judaism, Christianity, Hinduism, Buddhism, Islam, or the Baha’i Faith) as to the phenomenon of religion as a whole. All of these religions, with their infinite diversity of tradition, thought practice and understanding, form part of a greater whole that is “Religion”. That broader entity forms a single whole. Moreover that oneness of the greater whole carries direct implications for the way human beings relate to each other. Bahá’u’lláh writes:
That the divers communions of the earth, and the manifold systems of religious belief, should never be allowed to foster the feelings of animosity among men, is, in this Day, of the essence of the Faith of God and His Religion. These principles and laws, these firmly-established and mighty systems, have proceeded from one Source, and are rays of one Light. That they differ one from another is to be attributed to the varying requirements of the ages in which they were promulgated.
There can be no doubt whatever that the peoples of the world, of whatever race or religion, derive their inspiration from one heavenly Source, and are the subjects of one God. 
Or the following:
This is the changeless Faith of God, eternal in the past, eternal in the future.
Abdu’l Baha who repeatedly spoke of this principle sometimes observed in connection with it:
“reality is one and not multiple”
Yet as human beings we are infinitely diverse in our understanding of truth — all of us struggling to see more clearly through the limits of our perception and limits of our minds. None of us assured that our individual understanding at any particular time has grasped the truth or any part of it. In another passage Abdu’l Baha writes:
… when you meet those whose opinions differ from your own, do not turn away your face from them. All are seeking truth, and there are many roads leading thereto. Truth has many aspects, but it remains always and forever one. Do not allow difference of opinion, or diversity of thought to separate you from your fellowmen, or to be the cause of dispute, hatred and strife in your hearts. Rather, search diligently for the truth and make all men your friends.
The children of men are all brothers, and the prerequisites of brotherhood are manifold. Among them is that one should wish for one’s brother that which one wisheth for oneself. Therefore, it behoveth him who is the recipient of an inward or outward gift or who partaketh of the bread of heaven to inform and invite his friends with the utmost love and kindness. If they respond favourably, his object is attained; otherwise he should leave them to themselves without contending with them or uttering a word that would cause the least sadness. This is the undoubted truth, and aught else is unworthy and unbecoming.
An aspect not mentioned so far, is the central role of founders of religions – this is after all where religions come from. Most religions are identified with an identifiable figure who was the generator of that particular tradition. At one level they are all one and the same. At another they each have a specific mission.
In respect of his own mission Bahá’u’lláh states:
The All-Knowing Physician hath His finger on the pulse of mankind. He perceiveth the disease, and prescribeth, in His unerring wisdom, the remedy. Every age hath its own problem, and every soul its particular aspiration. The remedy the world needeth in its present-day afflictions can never be the same as that which a subsequent age may require. Be anxiously concerned with the needs of the age ye live in, and centre your deliberations on its exigencies and requirements.
(This article is the 12th in a series of what I hope will be 200 articles in 200 days for the 200th anniversary of the birth of Bahá’u’lláh. The anniversary is being marked around the world on 21 and 22 October 2017. The articles are simply my personal reflections on Bahá’u’lláh’s life and work. Responsibility for any inadequacies and inaccuracies are my own.)
Image Credit: Sulaymaniyah, Kurdistan, Iraq, near Tawela Village. Diyar Muhammad. Creative Commons Wikimedia