Will and Testament of 'Abdu'l-Baha - by 'Abdu'l-Baha

…the Will and Testament of 'Abdu'l-Bahá, that Charter of Bahá'u'lláh's New World Order, the offspring resulting from the mystic intercourse between Him Who is the Source of the Law of God and the mind of the One Who is the vehicle and interpreter of that Law. (Shoghi Effendi, God Passes By, p. xv)

The Charter which called into being, outlined the features and set in motion the processes of, this Administrative Order is none other than the Will and Testament of 'Abdu'l-Bahá, His greatest legacy to posterity, the brightest emanation of His mind and the mightiest instrument forged to insure the continuity of the three ages which constitute the component parts of His Father's Dispensation. (Shoghi Effendi, God Passes By)

… a document which confirms, supplements, and correlates the provisions of the Aqdas --
(Shoghi Effendi, The World Order of Baha'u'llah, p. 19)

It would, however, be helpful and instructive to bear in mind certain basic principles with reference to 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. (Shoghi Effendi, The World Order of Baha'u'llah, p. 3)

The Will and Testament of 'Abdu'l-Bahá, on the other hand, may be regarded as the offspring resulting from that mystic intercourse between Him Who had generated the forces of a God-given Faith and the One Who had been made its sole Interpreter and was recognized as its perfect Exemplar. (Shoghi Effendi, God Passes By, p. 325)

Statements such as these indicate that the full meaning of the Will and Testament of 'Abdu'l-Bahá, as well as an understanding of the implications of the World Order ushered in by that remarkable Document can be revealed only gradually to men's eyes, and after the Universal House of Justice has come into being. The friends are called upon to trust to time and to await the guidance of the Universal House 352 of Justice, which, as circumstances require, will make pronouncements that will resolve and clarify obscure matters. 
(Universal House of Justice, The Compilation of Compilations vol. I)

The contents of the Will of the Master is far too much for the present generation to comprehend. It needs at least a century of actual working before the treasures of wisdom hidden in it can be revealed. How can we at this stage and with our limited understanding denounce its spirit and purport.
(From a letter written on behalf of the Guardian to an individual believer, March 25, 1930) (Compilations, Lights of Guidance)

"What he considers, however, to be now of the utmost importance is for the believers, each and all, to cling firmly to the provisions of our beloved Master's Will and Testament, as by this means alone the unity of the Cause, and its safe and speedy growth can be maintained, safeguarded and insured. Such an absolute and unwavering fidelity to 'Abdu'l-Bahá's Will, and firm adherence to the principles of the Administrative Order is indeed incumbent upon every one of the friends, without any distinction whatever. Upon this basis alone the Faith can be safeguarded and flourish."
(From a letter written on behalf of the Guardian to the National Spiritual Assembly of the United States, July 18, 1938) (Compilations, Lights of Guidance)