Oneness of Religion

From the Writings of Baha'u'llah:
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. The difference between the ordinances under which they abide should be attributed to the varying requirements and exigencies of the age in which they were revealed. All of them, except a few which are the outcome of human perversity, were ordained of God, and are a reflection of His Will and Purpose.
    (Baha'u'llah: Gleanings, p. 217)

Know thou assuredly that the essence of all the Prophets of God is one and the same. Their unity is absolute. God, the Creator, saith: There is no distinction whatsoever among the Bearers of My Message. They all have but one purpose; their secret is the same secret. To prefer one in honor to another, to exalt certain ones above the rest, is in no wise to be permitted. Every true Prophet hath regarded His Message as fundamentally the same as the Revelation of every other Prophet gone before Him.
    (Baha'u'llah: Gleanings, pp. 78-79)

From the Writings and Utterances of `Abdu'l-Baha:
The foundations of the divine religions are one. If we investigate these foundations, we discover much ground for agreement, but if we consider the imitations of forms and ancestral beliefs, we find points of disagreement and division; for these imitations differ, while the sources and foundations are one and the same. That is to say, the fundamentals are conducive to unity, but imitations are the cause of disunion and dismemberment.
    (`Abdu'l-Baha: Promulgation of Universal Peace, p. 41)

Baha'u'llah promulgated the fundamental oneness of religion. He taught that reality is one and not multiple, that it underlies all divine precepts and that the foundations of the religions are, therefore, the same. Certain forms and imitations have gradually arisen. As these vary, they cause differences among religionists. If we set aside these imitations and seek the fundamental reality underlying our beliefs, we reach a basis of agreement because it is one and not multiple.
    (`Abdu'l-Baha: Promulgation of Universal Peace, p. 175)

The religion of God is one religion, but it must ever be renewed.
    (`Abdu'l-Baha: Selections ... `Abdu'l-Baha, p. 52)

Although the divine teachings are truth and reality, yet with the passage of time thick clouds envelop and obscure them. These clouds are imitations and superstitions; they are not the fundamentals. Then the Sun of Truth, the Word of God, arises again, shines forth once more in the glory of its power and disperses the enveloping darkness.
    (`Abdu'l-Baha: Promulgation of Universal Peace, p. 62)

The divine religions are like the progression of the seasons of the year. When the earth becomes dead and desolate and because of frost and cold no trace of vanished spring remains, the springtime dawns again and clothes everything with a new garment of life. The meadows become fresh and green, the trees are adorned with verdure and fruits appear upon them. Then the winter comes again, and all the traces of spring disappear. This is the continuous cycle of the seasons - spring, winter, then the return of spring. But though the calendar changes and the years move forward, each springtime that comes is the return of the springtime that has gone; this spring is the renewal of the former spring. Springtime is springtime, no matter when or how often it comes. The divine Prophets are as the coming of spring, each renewing and quickening the teachings of the Prophet Who came before Him. Just as all seasons of spring are essentially one as to newness of life, vernal showers and beauty, so the essence of the mission and accomplishment of all the Prophets is one and the same.
    (`Abdu'l-Baha: Promulgation of Universal Peace, pp. 126-127)

Baha'u'llah teaches that the foundations of the divine religion are one reality which does not admit of multiplicity or division. Therefore, the commandments and teachings of God are one. The religious differences and divisions which exist in the world are due to blind imitations of forms without knowledge or investigation of the fundamental divine reality which underlies all the religions. Inasmuch as these imitations of ancestral forms are various, dissensions have arisen among the people of religion. Therefore, it is necessary to free mankind from this subjection to blind belief by pointing the way of guidance to reality itself, which is the only basis of unity.
    (`Abdu'l-Baha: Promulgation of Universal Peace, p. 287)

...Baha'u'llah declared the necessity of peace among the nations and the reality of reconciliation between the religions of the world. He announced that the fundamental basis of all religion is one, that the essence of religion is human fellowship and that the differences in belief which exist are due to dogmatic interpretation and blind imitations which are at variance with the foundations established by the Prophets of God. He proclaimed that if the reality underlying religious teaching be investigated all religions would be unified, and the purpose of God, which is love and the blending of human hearts, would be accomplished.
    (`Abdu'l-Baha: Promulgation of Universal Peace, p. 354)

The essentials of the divine religion are one reality, indivisible and not multiple. It is one. And when through investigation we find it to be single, we have a basis for the oneness of the world of humanity.
    (`Abdu'l-Baha: Promulgation of Universal Peace, p. 42)

From the Writings of Shoghi Effendi:
The fundamental principle enunciated by Baha'u'llah ... is that religious truth is not absolute but relative, that Divine Revelation is a continuous and progressive process, that all the great religions of the world are divine in origin, that their basic principles are in complete harmony, that their aims and purposes are one and the same, that their teachings are but facets of one truth, that their functions are complementary, that they differ only in the nonessential aspects of their doctrines, and that their missions represent successive stages in the spiritual evolution of human society....
    (Shoghi Effendi: The Promised Day is Come, p. v)

The Faith standing identified with the name of Baha'u'llah disclaims any intention to belittle any of the Prophets gone before Him, to whittle down any of their teachings, to obscure, however slightly, the radiance of their Revelations, to oust them from the hearts of their followers, to abrogate the fundamentals of their doctrines, to discard any of their revealed Books, or to suppress the legitimate aspirations of their adherents. Repudiating the claim of any religion to be the final revelation of God to man, disclaiming finality for His own Revelation, Baha'u'llah inculcates the basic principle of the relativity of religious truth, the continuity of Divine Revelation, the progressiveness of religious experience. His aim is to widen the basis of all revealed religions and to unravel the mysteries of their scriptures. He insists on the unqualified recognition of the unity of their purpose, restates the eternal verities they enshrine, coordinates their functions, distinguishes the essential and the authentic from the nonessential and spurious in their teachings, separates the God-given truths from the priest-prompted superstitions, and on this as a basis proclaims the possibility, and even prophecies the inevitability, of their unification, and the consummation of their highest hopes.
    (Shoghi Effendi: The Promised Day is Come, p. 108)

One cannot call one World Faith superior to another, as they all come from God; they are progressive, each suited to certain needs of the time.
    (Shoghi Effendi: Directives of the Guardian, p. 40)