The Baha'i Faith teaches that there is one God: eternal, uncreated, the source of all things, and in essence unknowable. Human language and thought cannot reach the divine essence, only its attributes as they are mirrored in the Manifestations of God and in the created world. Baha'u'llah holds that the God worshipped under the many names of the religions, as Yahweh, as God, as Allah, as the Absolute, is one and the same Reality, differently addressed. This uncompromising unity is the root from which the other Baha'i teachings grow: because God cannot be known directly, revelation must be mediated through the Manifestations; and because there is one God and one revelation, there is finally one human family.
Baha'u'llah developed the doctrine at length in the Kitab-i-Iqan, the Book of Certitude, written in 1862 before his declaration of prophetic mission, arguing that every earlier scripture already points to this same hidden, unknowable God. Scholars of the Baha'i Faith, including Juan Cole, have traced the doctrine's vocabulary partly to the Islamic principle of tawhid and to Sufi and Shaykhi theology current in nineteenth-century Iran and Iraq, the milieu from which the Babi movement out of which Baha'u'llah's own mission emerged; Baha'is themselves hold the teaching to be an independent and renewed revelation of a truth underlying, rather than borrowed from, those earlier currents.