An introduction to the spiritual universe unveiled in the Kitáb-i-Aqdas would fail in its purpose if it did not acquaint the reader with the interpretive and legislative institutions that Bahá’u’lláh has indissolubly linked with the system of law thus revealed. At the foundation of this guidance lies the unique role which Bahá’u’lláh’s Writings--indeed the text of the Kitáb-i-Aqdas itself--confer on His eldest son, ‘Abdu’l-Bahá. This unique figure is at once the Exemplar of the pattern of life taught by His Father, the divinely inspired authoritative Interpreter of His Teachings and the Centre and Pivot of the Covenant which the Author of the Bahá’í­ Revelation made with all who recognize Him. The twenty-nine years of ‘Abdu’l-Bahá’s ministry endowed the Bahá’í­ world with a luminous body of commentary that opens multiple vistas of understanding on His Father’s purpose.

In His Will and Testament ‘Abdu’l-Bahá conferred the mantle of Guardian of the Cause and infallible Interpreter of its teachings upon His eldest grandson, Shoghi Effendi, and confirmed the authority and guarantee of divine guidance decreed by Bahá’u’lláh for the Universal House of Justice on all matters "which have not outwardly been revealed in the Book". The Guardianship and the Universal House of Justice can thus be seen to be, in the words of Shoghi Effendi, the "Twin Successors" of Bahá’u’lláh and ‘Abdu’l-Bahá. They are the supreme institutions of the Administrative Order which was founded and anticipated in the Kitáb-i-Aqdas and elaborated by ‘Abdu’l-Bahá in His Will.