The Book of Certitude, by Bahá'u'lláh, author of the Baha'i Revelation, is arguably one of the most important scriptural works in all of religious history. In it Bahá'u'lláh explains the underlying unity of the world's religions, describes the universality of the revelations humankind has received from the Prophets of God, explains their fundamental teachings, and elucidates allegorical passages from the New Testament and the Qur'an that have given rise to misunderstandings among religious leaders, practitioners, and the public. Written 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 Baha'i Faith.