The Kitab-i-Íqán, The Book of Certitude, is one of the most important scriptural works in all of the religious history. In it, Bahá'u'lláh gives a sweeping overview of religious truth, explaining the underlying unity of the world's religions, describing the universality of the revelations humankind has received from the Prophets of God, illuminating their fundamental teachings, and elucidating allegorical passages from the New Testament and the Qur'an that have given rise to misunderstanding among religious leaders, practitioners, and the public.

Revealed in the span of two days and two nights, The Book of Certitude is, in the words of its translator, Shoghi Effendi, "the most important book written on the spiritual significance" of the Bahá'í Faith.