Chief depository ... elements of that Divine Civilization,

The Will and Testament of ‘Abdu’l-Bahá, which, together with the Kitáb-i-Aqdas, constitutes the chief depository wherein are enshrined those priceless elements of that Divine Civilization, the establishment of which is the primary mission of the Bahá’í Faith. A study of the provisions of these sacred documents will reveal the close relationship that exists between them, as well as the identity of purpose and method which they inculcate. Far from regarding their specific provisions as incompatible and contradictory in spirit, every fair-minded inquirer will readily admit that they are not only complementary, but that they mutually confirm one another, and are inseparable parts of one complete unit. A comparison of their contents with the rest of Bahá’í sacred Writings will similarly establish the conformity of whatever they contain with the spirit as well as the letter of the authenticated writings and sayings of Bahá’u’lláh and ‘Abdu’l-Bahá. In fact, he who reads the Aqdas with care and diligence will not find it hard to discover that the Most Holy Book itself anticipates in a number of passages the institutions which ‘Abdu’l-Bahá ordains in His Will. By leaving certain matters unspecified and unregulated in His Book of Laws, Bahá’u’lláh seems to have deliberately left a gap in the general scheme of Bahá’í Dispensation, which the unequivocal provisions of the Master’s Will have filled. To attempt to divorce the one from the other, to insinuate that the Teachings of Bahá’u’lláh have not been upheld, in their entirety and with absolute integrity, by what ‘Abdu’l-Bahá has revealed in His Will, is an unpardonable affront to the unswerving fidelity that has characterized the life and labors of our beloved Master.

Shoghi Effendi "The World Order of Bahá’u’lláh"