Possibly the central principle behind the Bahá'í concept of the oneness of religion, progressive revelation asserts two important positions: 1) that all the major religions of the world are at least partially based on a divine revelation, conveyed to them by a Manifestation of God (q. v.); and 2) that the revelations brought by the Manifestations are not contradictory, but constitute a single, ongoing divine educational process for humanity.
1. Definition of Revelation
The Bahá'ís definition of revelation (Arabic wahí), like the Muslim definition, is often distinct from definitions used in other religions or from popular definitions. Bahá'ís make a sharp distinction between revelation--which is vouchsafed by God to only a very small group of humans--and inspiration (Arabic ilhám), which may come to any human being from various sources (such as God, departed souls, the mind, and the ego). The primary recipients of revelation are the Manifestations of God (or independent Prophets, as 'Abdu'l-Bahá calls them in Some Answered Questions, 164), individuals to whom God has revealed an entire system of spiritual, social, and ethical teachings. Some Manifestations identified in the Bahá'í scriptures are Abraham, Moses, Jesus, Muhammad, Zoroaster, Krishna, Buddha, the Báb, and Bahá'u'lláh.
But in addition to these individuals, the Bahá'í writings accept the Old Testament prophets--such as Isaiah, Jeremiah, and Ezekiel--as recipients of revelation, albeit a narrowly-focused revelation and not a complete spiritual, social, and ethical system. While the word "revelation" may be used to refer to the experiences of both groups of recipients, the Bahá'í scriptures note that the Manifestations receive their revelation "without an intermediary" (Some Answered Questions, 164) while the other prophets "are followers and promoters, for they are branches and are not independent" and "of themselves they have no power and might, except what they receive from the independent Prophets" (Some Answered Questions, 164, 165).
The question whether 'Abdu'l-Bahá is a recipient of revelation highlights the distinctive Bahá'í use of the word. Shoghi Effendi states that while 'Abdu'l-Bahá is definitely not a Manifestation of God, 'Abdu'l-Bahá possesses "superhuman knowledge and perfection" (World Order of Bahá'u'lláh, 134) and that He "gets His light, His inspiration and sustenance direct from the Fountainhead of the Bahá'í Revelation [Bahá'u'lláh]" (World Order of Bahá'u'lláh, 139). Such a description resembles the definition of revelation received by a dependent prophet such as Isaiah, but uses the word "inspiration" to describe the phenomenon instead of "revelation." Most likely the Bahá'í scriptures recognize six different related phenomena, even though they utilize only two words for them: the direct revelation received by an independent Prophet or Manifestation; the dependent revelation received by a lesser or dependent prophet; the dependent revelation or inspiration received by 'Abdu'l-Bahá; the inspiration received by Shoghi Effendi, and by the Universal House of Justice, which the Bahá'í writings state is infallible and unfailing; and the inspiration other persons receive.
Bahá'u'lláh described the nature of His revelation as follows:
Thou knowest full well that We perused not the books which men possess and We acquired not the learning current amongst them, and yet whenever We desire to quote the sayings of the learned and of the wise, presently there will appear before the face of thy Lord in the form of a tablet all that which hath appeared in the world and is revealed in the Holy Books and Scriptures. Thus do We set down in writing that which the eye perceiveth (Lawh-i-Hikmat or "Tablet of Wisdom," in Tablets of Bahá'u'lláh revealed after the Kitáb-i-Aqdas, 149).Descriptions of Bahá'u'lláh's reception of revelation note the power felt by those present. Hájí Mírzá Habíbu'lláh Afnán stated that
The flow of verses from the heaven of Revelation was swift. It was indeed like unto a fast-billowing ocean. Mírzá Áqá Ján [Bahá'u'lláh's amanuensis] wrote as quickly as he could--so quickly that the pen at times jumped out of his hand. He would immediately take up another pen. There were times when he could not keep up and would say: 'I am incapable of writing.' Then the Blessed Perfection [Bahá'u'lláh] would repeat what He had spoken" (quoted in Balyuzi, Bahá'u'lláh: King of Glory, 413).Clearly revelation, as experienced by Bahá'u'lláh, was not simply an ordinary form of literary composition.
2. The universality of divine revelation
Bahá'u'lláh makes it clear that divine revelation has not been confined to a particular period of human history. Rather, He states that "the Manifestations of His Divine Glory. . . have been sent down from time immemorial, and been commissioned to summon mankind to the one true God. That the names of some of them are forgotten and the records of their lives lost is to be attributed to the disturbances and changes that have overtaken the world" (Gleanings from the Writings of Bahá'u'lláh, 174). This statement is important because it makes it clear that religions other than the ones possessing known Manifestations may have been divinely established. In addition to the historic figures already listed, Bahá'u'lláh mentions Húd and Sálih--legendary figures who appeared to Arab tribes, and who are also mentioned in the Qur'án--as Manifestations. They may be seen as examples of Manifestations to primal religions, as the religions of the tribal peoples of the world are often called by religious scholars. While Bahá'u'lláh was not asked about other Manifestations, such as to the African, Chinese, native American, and ancient Indo-European peoples, it is reasonable to assume, based on Bahá'u'lláh's statement above, that Manifestations came to them as well. Based on a statement of 'Abdu'l-Bahá that "in cycles gone by. . . continents remained widely divided, nay even among the people of one and the same continent association and interchange of thought were wellnigh impossible" (Selections from the Writings of 'Abdu'l-Bahá, 31) the Research Department of the Universal House of Justice has concluded that "it would appear possible that Manifestations of God have lived simultaneously in different areas of the globe" (Research Department memorandum to the Universal House of Justice titled "Questions Relayed by the Spiritual Assembly of Mitcham," dated 24 May 1988).
The folklore of most ethnic groups contain the story of an individual who brought civilization (technological, spiritual, social, and moral knowledge) to that group from God or the gods; religious scholars often refer to these individuals as culture heroes. Possibly some stories of culture heroes preserve accounts of ancient Manifestations. No where do the Bahá'í writings say that Manifestations must be male, and many culture heroes are female.
While some primal religions may have had their own Manifestations, other religions may not. Shoghi Effendi explicitly stated that Confucius and Lao-Tzu were not Manifestations, thereby relegating any Chinese Manifestation to prehistory. Mahavira (c. 600 B.C.), the founding figure of Jainism, is not mentioned at all in the Bahá'í scriptures and thus Bahá'ís do not know whether he was a Manifestation. The fact that he lived in India almost the same time as the Buddha argues against, but does not preclude the possibility. Guru Nanak Sahib, founder of Sikhism (c. 1500 C.E.), lived between the time of Muhammad and the Báb and the Bahá'í writings refer to that period as the dispensation of Muhammad, thus implying that it had no Manifestation. Presumably Bahá'ís would view Sikhism as a religion based on inspiration, not revelation, and drawing off the Indian and Middle Eastern revelations for its teachings. There is no official Bahá'í position about Mani (216-76 C.E.), the founder of Manichaeism.
3. The progressive nature of revelation
'Abdu'l-Bahá noted that "the world of existence is progressive. It is subject to development and growth" (Promulgation of Universal Peace, 378). Since revelation is part of the world, thus it must also develop and change; and the progressive revealing of divine truth is one of the main causes for the progress of human civilization. The Bahá'í scriptures assert that while all revelations bring eternal and unchanging teachings--such as teachings about one's relation to the divine, and moral fundamentals such as doing unto others as one would have them do to oneself--each also brings truths suited to its own time and place. Thus Abraham focused much of His mission on teaching the existence of one God; Moses was able to move beyond assertion of the existence of one God and reveal laws that established the relationship between God and Israelite society; Jesus revealed about the individual relationship to God, thereby broadening and deepening the relationship established by Moses's social laws; Muhammad was able to integrate the two into a holistic system for personal and social relationship to the divine; and Bahá'u'lláh was able to update the personal and social relationship for the modern age. Because each revelation has a temporal aspect, it must eventually be superseded by a new revelation. The Bahá'í scriptures state that each revelation builds on the previous revelation known in that part of the world, and in turn becomes the foundation for a subsequent revelation.
The Bahá'í scriptures do not assert that each revelation is overtly influenced by all the ones that went before it somewhere on earth; if that were the case, Jesus would refer to the Buddha and His teachings, and He clearly does not. Rather, in the Middle East one can identify a chain of Manifestations whose teachings are in historical continuity: Abraham, Moses, Jesus, and Muhammad. Zoroaster's revelation, which occurred in Iran and probably after Moses, had some influence on the last two. India has a short chain of known Manifestations--Krishna and the Buddha--both of whom built their teachings on earlier Indian religious traditions. The Bahá'í writings state that all the previous revelations find their fulfillment and completion in the revelation of Bahá'u'lláh.
4. Continuity and Discontinuity in Religion
The Bahá'í claim that all religions ultimately can be traced back to God asserts a fundamental unity and continuity of the religions, but it does not ignore the discontinuities and differences. Religions are bewilderingly diverse and contain many apparent contradictions. Bahá'ís resolve apparent contradictions among them in a variety ways.
The first is to acknowledge that religions contain a major component of human interpretation, as well as revelation. To take Christianity and Islam as examples, many of the doctrines accepted by Christians and Muslims constitute interpretations of their revelations and are not the revelations themselves. Thus most Muslims maintain that Jesus was not crucified, but one who looked like Him was instead, based on their understanding of Qur'án 4:156. Shoghi Effendi, however, states that the Qur'ánic passage indicates that the spiritual reality of Christ was beyond crucifixion, not that His body escaped such a fate (Lights of Guidance, 1646, 1652, 1669); this resolves an apparent contradiction between Islam and Christianity. Most Christians maintain that God is triune based on an interpretation of a few references to the Father, Son, and Holy Spirit in the New Testament; the Bahá'í scriptures, however, view trinitarian statements as metaphors and not as ontological assertions, thus strictly maintain the unity of God, like Islam (for example, Some Answered Questions, 113-15).
A second approach Bahá'ís take to explain the contradictions between religions is that there are temporal aspects of revelation that must change. 'Abdu'l-Bahá cites the marriage and divorce laws of Moses and Jesus as a prime example (Promulgation of Universal Peace, 365).
A third approach is to acknowledge the inadequacy of historical records and the transmission of the original teachings. Thus 'Abdu'l-Bahá stated that the Buddha "established the Oneness of God, but later the original principles of His doctrines gradually disappeared, and ignorant customs and ceremonials arose and increased" (Some Answered Questions, 165). Shoghi Effendi, in various letters, notes that Islam is the fullest revelation before the Bahá'í Faith, and that the Bible and the scriptures of Hinduism and Buddhism are not literal records of divine revelation, and thus we cannot be sure of the accuracy of many of the events and teachings they describe (Lights of Guidance, 1670, 1660, 1684, 1692, 1693, 1696). Shoghi Effendi also emphasizes the importance of relying on the work of historians (Lights of Guidance, 1692, 1696), suggesting that scholarly efforts to reconstruct the historical Jesus or Moses or Buddha and their teachings can be valuable to Bahá'ís.
Finally, even where revelations are preserved in detail there are numerous examples of difference between them. A comparison of the Old and New Testaments, or the Bábí and Bahá'í sacred writings, will reveal many. While some of the differences are attributable to the different circumstances of revelation, the personal experiences of the Manifestations may have played a role as well; if nothing else Their experiences shaped their literary style and dictated many of the issues They addressed and the examples They gave. Thus progressive revelation should not be seen as a detailed theory that explains all the features of religions, but as a framework within which Bahá'ís work to understand the different religions and their diversity.
 This presumably means a human intermediary, since nonhuman intermediaries are often mentioned in scripture; Bahá'u'lláh received revelation from a Maid of Heaven (God Passes By, 101-02) and Muhammad from the archangel Gabriel.
 I am indebted to Mr. Christopher Buck for drawing my attention to this research memorandum.
 To give a few examples: Prometheus, who, in Greek mythology, stole fire from the gods and brought it to humanity and was punished by the gods for his actions; Guang Di or the Yellow Emperor, the legendary first emperor of China who brought the arts of civilization from heaven; the White Buffalo Calf Woman, who brought the sacred pipe and other central sacred rites to the Lakota Sioux from Wakan Tanka (the Great Incomprehensible).
 Perhaps Bahá'ís can view Mahavira as a figure akin to John the Baptist; one who lived immediately before a Manifestation, anticipating some of the Manifestation's teachings, and preparing the population for Him.