The idea of Progressive Revelation

Bahá'u'lláh teaches that just as there is only one God, there is also only one religion. All the great Prophets have taught this same one religion. There is, Bahá'u'lláh tells us, no exclusive salvation for the Hindu, the Jew, the Zoroastrian, the Buddhist, the Christian, the Muslim, or the Bahá'í. The Bahá'í Faith is not a sect, but an independent religion. The famous historian Arnold Toynbee was asked about Bahá'u'lláh's Faith. He replied: 'My opinion is that (1) Bahá'ísm is undoubtedly a religion, (2) Bahá'ísm is an independent religion, on a par with Islam, Christianity, and the other recognized world religions. Bahá'ísm is not a sect of some other religion; it is a separate religion, and it has the same status of other recognized religions. This opinion is based both on study and on personal acquaintance with Bahá'ís.' [Bahá'í journal, November, 1959.] All these pure and holy Faiths are part of the one eternal religion of God which goes on for ever. No single religion is the one exclusive Faith, or the final outpouring of truth from Almighty God. Each religion is true, is beautiful, is valid. It is the one Message from God for that age in which it appears. It is the only truth for that particular age, yet it is not final. It is but one part of a single, great, progressive, never-ending Religion of God which has no beginning and will have no end. Though the Word (Holy Spirit) of God is one, the Speakers (Messengers) of this Word are many. It is the one light in many lamps. The golden rule can be found in all the great religions of the world:
Hinduism: The true rule is to do by the things of others as you do by your own.
Judaism: Whatever you do not wish your neighbor to do to you do not to him.
Zoroastrianism: As you do you will be done by.
Buddhism: One should seek for others the happiness one desires for one's self.
Christianity: Therefore, all things whatsoever you would that men should do to you, do even so to them.
Islam: Let none of you treat a brother in a way he himself would dislike to be treated.
Bahá'í Faith: If thou regardest Mercy, look not to that which benefits thyself; but to that which will benefit thy fellowmen. If thou regardest Justice, choose thou for others that which thou choosest for thyself.
The Teachings of Bahá'u'lláh liken religion to the growth of a plant. Dr. Esslemont who spent many years studying the Teachings of Bahá'u'lláh states it in this way: 'The religion of God is the One Religion, and all the Prophets have taught it, but it is a living and growing thing, not lifeless and unchanging. In the teaching of Moses we see the bud; in that of Christ the flower; in that of Bahá'u'lláh the fruit. The flower does not destroy the bud, nor does the fruit destroy the flower. It destroys not, but fulfils. The bud-scales must fall in order that the flower may bloom, and the petals must fall that the fruit may grow and ripen. Were the bud-scales or the petals wrong or useless, then, that they had to be discarded? No, both in their time were right and necessary; without them there could have been no fruit. So it is with the various prophetic teachings; the externals change from age to age, but each revelation is the fulfilment of its predecessors; they are not separate or incongruous, but different stages in the life history of One Religion, which has in turn been revealed as seed, as bud, as flower, and now enters on the stage of fruition.

~ Bahá'u'lláh and the New Era, Esslemont, Ch. VIII.