Foremost among the priceless treasures cast forth from the billowing ocean of Bahá'u'lláh's Revelation ranks the Kitab-i-Iqan (Book of Certitude), revealed within the space of two days and two nights, in the closing years of that period (1278 A.H. -- 1862 A.D.). It was written in fulfillment of the prophecy of the Bab, Who had specifically stated that the Promised One would complete the text of the unfinished Persian Bayan, and in reply to the questions addressed to Baha'u'llah by the as yet unconverted maternal uncle of the Bab, Haji Mirza Siyyid Muhammad, while on a visit, with his brother, Haji Mirza Hasan-'Ali, to Karbila. A model of Persian prose, of a style at once original, chaste and vigorous, and remarkably lucid, both cogent in argument and matchless in its irresistible eloquence, this Book, setting forth in outline the Grand Redemptive Scheme of God, occupies a position unequalled by any work in the entire range of Baha'i literature, except the Kitab-i-Aqdas, Baha'u'llah's Most Holy Book. Revealed on the eve of the declaration of His Mission, it proffered to mankind the "Choice Sealed Wine," whose seal is of "musk," and broke the "seals" of the "Book" referred to by Daniel, and disclosed the meaning of the "words" destined to remain "closed up" till the "time of the end."
Within a compass of two hundred pages it proclaims unequivocally the existence and oneness of a personal God, unknowable, inaccessible, the source of all Revelation, eternal, omniscient, omnipresent and almighty; asserts the relativity of religious truth and the continuity of Divine Revelation; affirms the unity of the Prophets, the universality of their Message, the identity of their fundamental teachings, the sanctity of their scriptures, and the twofold character of their stations; denounces the blindness and perversity of the divines and doctors of every age; cites and elucidates the allegorical passages of the New Testament, the abstruse verses of the Qur'an, and the cryptic Muhammadan traditions which have bred those age-long misunderstandings, doubts and animosities that have sundered and kept apart the followers of the world's leading religious systems; enumerates the essential prerequisites for the attainment by every true seeker of the object of his quest; demonstrates the validity, the sublimity and significance of the Bab's Revelation; acclaims the heroism and detachment of His disciples; foreshadows, and prophesies the world-wide triumph of the Revelation promised to the people of the Bayan; upholds the purity and innocence of the Virgin Mary; glorifies the Imams of the Faith of Muhammad; celebrates the martyrdom, and lauds the spiritual sovereignty, of the Imam Husayn; unfolds the meaning of such symbolic terms as "Return," "Resurrection," "Seal of the Prophets" and "Day of Judgment"; adumbrates and distinguishes between the three stages of Divine Revelation; and expatiates, in glowing terms, upon the glories and wonders of the "City of God," renewed, at fixed intervals, by the dispensation of Providence, for the guidance, the benefit and salvation of all mankind. Well may it be claimed that of all the books revealed by the Author of the Baha'i Revelation, this Book alone, by sweeping away the age-long barriers that have so insurmountably separated the great religions of the world, has laid down a broad and unassailable foundation for the complete and permanent reconciliation of their followers.
(Shoghi Effendi, God Passes By, pp. 138-139)
Shoghi Effendi adds that this Book is one of Baha'u'llah's “outstanding contributions to the world's religious literature” occupying a position of “unsurpassed preeminence among the doctrinal ... writings of the Author of the Baha'i Dispensation”. (Shoghi Effendi, God Passes By, p. 140)