Overview:
For every era hath a spirit; the spirit of this illumined era lieth in the teachings of Baha’u’llah. For these lay the foundation of the oneness of the world of humanity and promulgate universal brotherhood. They are founded upon the unity of science and religion and upon investigation of truth. They uphold the principle that religion must be the cause of amity, union and harmony among men. They establish the equality of both sexes and propound economic principles which are for the happiness of individuals. They diffuse universal education, that every soul may as much as possible have a share of knowledge. They abrogate and nullify religious, racial, political, patriotic and economic prejudices and the like. Those teachings that are scattered throughout the Epistles and Tablets are the cause of the illumination and the life of the world of humanity. Whoever promulgateth them will verily be assisted by the Kingdom of God.
(`Abdu’l-Baha: Selections … `Abdu’l-Baha, p. 109)

The Baha’i Faith upholds the unity of God, recognizes the unity of His Prophets, and inculcates the principle of the oneness and wholeness of the entire human race. It proclaims the necessity and the inevitability of the unification of mankind, asserts that it is gradually approaching, and claims that nothing short of the transmuting spirit of God, working through His chosen Mouthpiece in this day, can ultimately succeed in bringing it about. It, moreover, enjoins upon its followers the primary duty of an unfettered search after truth, condemns all manner of prejudice and superstition, declares the purpose of religion to be the promotion of amity and concord, proclaims its essential harmony with science, and recognizes it as the foremost agency for the pacification and the orderly progress of human society….
(Shoghi Effendi: The Promised Day is Come, pp. v-vi)

It [the Baha’i Faith] is free from any form of ecclesiasticism, has neither priesthood nor rituals, and is supported exclusively by voluntary contributions made by its avowed adherents. Though loyal to their respective governments, though imbued with the love of their own country, and anxious to promote at all times, its best interests, the followers of the Baha’i Faith, nevertheless, viewing mankind as one entity, and profoundly attached to its vital interests, will not hesitate to subordinate every particular interest, be it personal, regional or national, to the over-riding interests of the generality of mankind, knowing full well that in a world of interdependent peoples and nations the advantage of the part is best to be reached by the advantage of the whole, and that no lasting result can be achieved by any of the component parts if the general interests of the entity itself are neglected….
(Shoghi Effendi: The Promised Day is Come, pp. vi-vii)

The Oneness of Mankind:
Of the principles enshrined in these Tablets the most vital of them all is the principle of the oneness and wholeness of the human race, which may well be regarded as the hall-mark of Baha’u’llah’s Revelation and the pivot of His teachings. Of such cardinal importance is this principle of unity that it is expressly referred to in the Book of His Covenant, and He unreservedly proclaims it as the central purpose of His Faith.
(Shoghi Effendi: God Passes By, pp. 216-217)

The principle of the Oneness of Mankind – the pivot round which all the teachings of Baha’u’llah revolve – is no mere outburst of ignorant emotionalism or an expression of vague and pious hope. Its appeal is not to be merely identified with a reawakening of the spirit of brotherhood and good-will among men, nor does it aim solely at the fostering of harmonious cooperation among individual peoples and nations. Its implications are deeper, its claims greater than any which the Prophets of old were allowed to advance. Its message is applicable not only to the individual, but concerns itself primarily with the nature of those essential relationships that must bind all the states and nations as members of one human family. It does not constitute merely the enunciation of an ideal, but stands inseparably associated with an institution adequate to embody its truth, demonstrate its validity, and perpetuate its influence. It implies an organic change in the structure of present-day society, a change such as the world has not yet experienced. It constitutes a challenge, at once bold and universal, to outworn shibboleths of national creeds – creeds that have had their day and which must, in the ordinary course of events as shaped and controlled by Providence, give way to a new gospel, fundamentally different from, and infinitely superior to, what the world has already conceived. It calls for no less than the reconstruction and the demilitarization of the whole civilized world – a world organically unified in all the essential aspects of its life, its political machinery, its spiritual aspiration, its trade and finance, its script and language, and yet infinite in the diversity of the national characteristics of its federated units.
It represents the consummation of human evolution – an evolution that has had its earliest beginnings in the birth of family life, its subsequent development in the achievement of tribal solidarity, leading in turn to the constitution of the city-state, and expanding later into the institution of independent and sovereign nations.
The principle of the Oneness of Mankind, as proclaimed by Baha’u’llah, carries with it no more and no less than a solemn assertion that attainment to this final stage in this stupendous evolution is not only necessary but inevitable, that its realization is fast approaching, and that nothing short of a power that is born of God can succeed in establishing it.
(Shoghi Effendi: World Order of Baha’u’llah, pp. 42-43)

Let there be no misgivings as to the animating purpose of the world-wide Law of Baha’u’llah. Far from aiming at the subversion of the existing foundations of society, it seeks to broaden its basis, to remold its institutions in a manner consonant with the needs of an ever-changing world. It can conflict with no legitimate allegiances, nor can it undermine essential loyalties. Its purpose is neither to stifle the flame of a sane and intelligent patriotism in men’s hearts, nor to abolish the system of national autonomy so essential if the evils of excessive centralization are to be avoided. It does not ignore, nor does it attempt to suppress, the diversity of ethnical origins, of climate, of history, of language and tradition, of thought and habit, that differentiate the peoples and nations of the world. It calls for a wider loyalty, for a larger aspiration than any that has animated the human race. It insists upon the subordination of national impulses and interests to the imperative claims of a unified world. It repudiates excessive centralization on one hand, and disclaims all attempts at uniformity on the other. Its watchword is unity in diversity…
(Shoghi Effendi: World Order of Baha’u’llah, pp. 41-42)

The Oneness of Religion:
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)

Indeed, the essential prerequisites of admittance into the Baha’i fold of Jews, Zoroastrians, Hindus, Buddhists, and the followers of other ancient faiths, as well as of agnostics and even atheists, is the wholehearted and unqualified acceptance by them all of the divine origin of both Islam and Christianity, of the Prophetic functions of both Muhammad and Jesus Christ, of the legitimacy of the institution of the Imamate, and of the primacy of St. Peter, the Prince of the Apostles. Such are the central, the solid, the incontrovertible principles that constitute the bedrock of Baha’i belief, which the Faith of Baha’u’llah is proud to acknowledge, which its teachers proclaim, which its apologists defend, which its literature disseminates, which its summer schools expound, and which the rank and file of its followers attest by both word and deed.
(Shoghi Effendi: The Promised Day is Come, p. 110)

The Oneness of God:
He [God] is, and hath from everlasting been, one and alone, without peer or equal, eternal in the past, eternal in the future, detached from all things, ever-abiding, unchangeable, and self-subsisting. He hath assigned no associate unto Himself in His Kingdom, no counsellor to counsel Him, none to compare unto Him, none to rival His glory.
(Baha’u’llah: Gleanings, p. 192)

Baha’u’llah appeared upon the horizon of the East, proclaiming the oneness of God and the unity of the world of humanity. He promulgated the teaching that all mankind are the servants of one God; that all have come into being through the bestowal of the one Creator; that God is kind to all, nurtures, rears and protects all, provides for all and extends His love and mercy to all races and people.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 174)

The reality of Divinity is sanctified above this degree of knowing and realization. It has ever been hidden and secluded in its own holiness and sanctity above our comprehending. Although it transcends our realization, its lights, bestowals, traces and virtues have become manifest in the realities of the Prophets, even as the sun becomes resplendent in various mirrors. These holy realities are as reflectors, and the reality of Divinity is as the sun, which, although it is reflected from the mirrors, and its virtues and perfections become resplendent therein, does not stoop from its own station of majesty and glory and seek abode in the mirrors; it remains in its heaven of sanctity. At most it is this: that its lights become manifest and evident in its mirrors or manifestations. Therefore, its bounty proceeding from them is one bounty, but the recipients of that bounty are many. This is the unity of God; this is oneness – unity of Divinity, holy above ascent or descent, embodiment, comprehension or idealization – divine unity. The Prophets are its mirrors; its lights are revealed through Them; its virtues become resplendent in Them, but the Sun of Reality never descends from its own highest point and station. This is unity, oneness, sanctity;…
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 193)

Place in History:
The Faith of Baha’u’llah should indeed be regarded, if we wish to be faithful to the tremendous implications of its message, as the culmination of a cycle, the final stage in a series of successive, of preliminary and progressive revelations. These, beginning with Adam and ending with the Bab, have paved the way and anticipated with an ever-increasing emphasis the advent of that Day of Days in which He Who is the Promise of All Ages should be made manifest.
(Shoghi Effendi: World Order of Baha’u’llah, p. 103)

The Revelation of Baha’u’llah, whose supreme mission is none other but the achievement of this organic and spiritual unity of the whole body of nations, should, if we be faithful to its implications, be regarded as signalizing through its advent the coming of age of the entire human race. It should be viewed not merely as yet another spiritual revival in the ever-changing fortunes of mankind, not only as a further stage in a chain of progressive Revelations, nor even as the culmination of one of a series of recurrent prophetic cycles, but rather as marking the last and highest stage in the stupendous evolution of man’s collective life on this planet. The emergence of a world community, the consciousness of world citizenship, the founding of a world civilization and culture – all of which must synchronize with the initial stages in the unfoldment of the Golden Age of the Baha’i Era – should, by their very nature, be regarded, as far as this planetary life is concerned, as the furthermost limits in the organization of human society, though man, as an individual, will, nay must indeed as a result of such a consummation, continue indefinitely to progress and develop.
(Shoghi Effendi: World Order of Baha’u’llah, p. 163)

Prophets of God in the Future:
After Baha’u’llah many Prophets will, no doubt, appear but they will be under His Shadow. Although they may abrogate the laws of this Dispensation in accordance with the needs and requirements of the age in which they appear, they nevertheless draw their spiritual force from this mighty Revelation. The Faith of Baha’u’llah constitutes, indeed, the stage of maturity in the development of mankind.
(Shoghi Effendi: Directives of the Guardian, p. 61)

The Dispensation of Baha’u’llah will last until the coming of the next Manifestation of God, Whose advent will not take place before at least “a full thousand years” will have elapsed.
(The Universal House of Justice: Aqdas: Notes, p. 195)