Progressive Revelation is the theory that provides the basis for the Bahá'í belief in the unity of religion. It maintains that all of the world's major religions come from the same divine source and that they do not contradict one another, but are all part of one divine plan for the spiritual education of humanity.
Bahá'u'lláh explains that each Manifestation of God teaches according to the needs of His age:
"No man, however acute his perception, can ever hope to reach the heights which the wisdom and understanding of the Divine Physician have attained. Little wonder, then, if the treatment prescribed by the physician in this day should not be found to be identical with that which he prescribed before. How could it be otherwise when the ills affecting the sufferer necessitate at every stage of his sickness a special remedy? In like manner, every time the Prophets of God have illumined the world with the resplendent radiance of the Day Star of Divine knowledge, they have invariably summoned its peoples to embrace the light of God through such means as best befitted the exigencies of the age in which they appeared." 1
`Abdu'l-Bahá further states that there are two kinds of religious laws: the essential, which never change and are common among all religions, and the non-essential, which are established to accomodate the time and place in which they appear.
...each of the divine religions contains two kinds of laws or ordinances. One division concerns the world of morality and ethical institutions. These are the essential ordinances. They instill and awaken the knowledge and love of God, love for humanity, the virtues of the world of mankind, the attributes of the divine kingdom, rebirth and resurrection from the kingdom of nature. These constitute one kind of divine law which is common to all and never subject to change. From the dawn of the Adamic cycle to the present day this fundamental law of God has continued changeless. This is the foundation of divine religion.
The second division comprises laws and institutions which provide for human needs and conditions according to exigencies of time and place... For example during the time of His Holiness Moses...divorce was permissible. During the cycle of His Holiness Christ inasmuch as divorce was not in conformity with the time and conditions His Holiness Jesus Christ abrogated it. In the cycle of Moses plurality of wives was permissible but during the time of His Holiness Christ the exigency which had sanctioned it did not exist, therefore it was forbidden. His Holiness Moses lived in the wilderness and desert of Sinai; therefore his ordinances and commandments were in conformity with those conditions. The penalty for theft was to cut off a man's hand. An ordinance of this kind was in keeping with desert life but not compatible with conditions of the present day.2
According to Bahá'í teachings, new revelations are necessary when followers of religions stray from the original teachings of their faiths.
The founder of Buddhism was a wonderful soul. He established the Oneness of God, but later the original principles of His doctrines gradually disappeared, and ignorant customs and ceremonials arose and increased until they finally ended in the worship of statues and images.3
In this way, the unity of the religions of the world is upheld, and apparent differences between them are ascribed to human corruption and ignorance of the true foundations of religion.
Bahá'í teachings also indicate that each revelation is made with respect to the maturity of humankind at the time of the coming of each Manifestation. An analogy is drawn between humanity and a child in school. As the child progresses to each new grade, he gets a new teacher who reinforces what previous teachers have taught and introduces new things which the child is now ready to comprehend. The level of the lessons does not depend on the knowledge of the teacher, but on the capacity of the child.
Based on the concept of Progressive Revelation, Bahá'ís believe that the world's religions together represent "the changeless Faith of God, eternal in the past, eternal in the future." 4
1. Gleanings from the Writings of Bahá'u'lláh, p80
2. `Abdu'l-Bahá, Foundations of World Unity, p79-83
3. `Abdu'l-Bahá, Some Answered Questions, p164-167
4. Bahá'u'lláh, The Kitáb-I-Aqdas, p85