The oneness of religion is progressive revelation seen from the other side. If one God has sent a succession of Manifestations, then the religions they founded are not many religions but one religion revealed in stages. Baha'i teaching distinguishes two layers in every revealed faith. The essential teachings, the knowledge and love of God, the reality and immortality of the soul, the summons to justice, honesty and compassion, are one and unchanging across them all. The social teachings, laws of marriage, diet, ritual and civil order, are provisional, given for a particular people and age and meant to change.
Where the religions appear to contradict one another, Baha'is attribute the conflict to human addition, to misreading, or to the lapsing of time-bound laws, not to their divine origin. This is neither a claim that the faiths are already the same nor a mixing of them, but the assertion that they are chapters of a single unfolding book.