Progressive Revelation

By Progressive Revelation we mean that over the course of human existence, God has spoken to us many times in many different parts of the world.  In each such Revelation, God has spoken to us through a human Messenger, albeit one specially prepared to deliver the message.  Each of the Revelations provided the knowledge and spiritual energy that was needed for further human development at that time and place.
On this topic, Baha'u'llah (Founder of the Baha'i Faith) wrote:
“The All-Knowing Physician hath His finger on the pulse of mankind. He perceiveth the disease, and prescribeth, in His unerring wisdom, the remedy. Every age hath its own problem, and every soul its particular aspiration. The remedy the world needeth in its present-day afflictions can never be the same as that which a subsequent age may require. Be anxiously concerned with the needs of the age ye live in, and centre your deliberations on its exigencies and requirements.”
Our history records the names of a few of these Messengers.  Adam, Krishna, Abraham, Buddha, Zoroaster, Moses, Christ, Mohammad, The Bab (predecessor of Baha'u'llah) and Baha'u'llah.  We call them Manifestations of God in that they make manifest to us the aspects and attributes of God that we humans can comprehend.
We see the Messengers from Adam through Mohammad as encompassing a cycle of prophesy.  They all intimated that in the future all God's children would be united in a common faith.  One of Mohammad's titles is Seal of the Prophets signifying the end of this prophetic cycle.  The Bab (meaning The Gate) stands between the cycle of prophecy and the cycle of fulfillment.  Baha'u'llah begins a new cycle of Messengers to fulfill God's promises to us.