Baha'is affirm the essential unity of all the great religions, that the world's major faiths spring from one God and form successive chapters of a single, unfolding revelation. On this teaching of progressive revelation, God has sent a series of Messengers (among them Abraham, Moses, Krishna, the Buddha, Zoroaster, Jesus, Muhammad, the Bab, and Baha'u'llah), each renewing the eternal truths and advancing them to meet the needs of a new age. The differences among religions are attributed to the varying times, places, and capacities they addressed, while their spiritual core is held to be one.
Baha'u'llah set out the doctrine at length in the Kitab-i-Iqan (Book of Certitude, 1862), arguing from the Bible and the Quran that each of God's Messengers, termed Manifestations of God, has been rejected in his own time by the very religious authorities who claimed to await him, a pattern the book reads as continuing in the response to the Bab and to Baha'u'llah himself. The claim is not accepted by the religions it names as predecessors: mainstream Sunni and Shia Islam holds Muhammad the final prophet and treats any claim to further prophecy as incompatible with that finality, a position that has led some Muslim-majority states to treat the Baha'i Faith as heretical rather than as a legitimate independent religion, and Christian orthodoxy similarly holds the revelation in Christ complete rather than awaiting a further Messenger.