Towards the end of His life, ‘Abdu’l-Bahá wrote a will and testament in which He appointed a successor, who would be the Guardian of the Cause of Bahá’u’lláh, and also an authorized interpreter of His Writings. This successor was ‘Abdu’l-Bahá’s grandson, Shoghi Effendi.
“O my loving friends! After the passing away of this wronged one, it is incumbent upon the Aghṣán (Branches), the Afnán (Twigs) of the Sacred Lote-Tree, the Hands (pillars) of the Cause of God and the loved ones of the Abha Beauty to turn unto Shoghi Effendi—the youthful branch branched from the two hallowed and sacred Lote-Trees and the fruit grown from the union of the two offshoots of the Tree of Holiness,—as he is the sign of God, the chosen branch, the Guardian of the Cause of God, he unto whom all the Aghṣán, the Afnán, the Hands of the Cause of God and His loved ones must turn. He is the Interpreter of the Word of God and after him will succeed the first-born of his lineal descendants.” (‘Abdu’l-Bahá, The Will and Testament, p. 11)
“Shoghi Effendi, Guardian of the Bahá’í Faith, and appointed Expounder of the Teachings of Bahá’u’lláh, its Founder, is the one human being in all history, past, present or future, to exercise the greatest influence on the ultimate shape and modus operandi of the social order of the world. He is the one who understood the vision of his great-grandfather Bahá’u’lláh and his grandfather ‘Abdu’l-Bahá—respectively the Revealer and the Interpreter of teachings destined to reshape the divisive society of the present world and usher in an era of universal peace—and applied Their doctrines in practical terms to the organization of such a future state of society. There cannot be, on this planet, a greater social or political unit than World Order. It is Shoghi Effendi who, while not the architect of that consummation, is certainly its chief builder and engineer. He laid the foundations of the Administrative Order of a Faith which, as it develops, will come—as he stated—”to be regarded not only as the nucleus but the very pattern of the New World Order destined to embrace in the fullness of time the whole of mankind.” (David Hofman, in the forward of the book The Guardian of the Bahá’í Faith by Ruhiyyih Khanum, p. vii)
“The Master’s grandson had been sublimed by the forces released in His Testament into the Guardian of the Faith; belonging to the sovereign caste of his divine Forefathers, he was himself a sovereign. To the primacy conferred by ties of consanguinity had been added the powers of infallible guidance with which the operation of God’s Covenant had invested him. Shoghi Effendi’s divine and indefeasible right to assume the helm of the Cause of God had been fully vindicated through thirty-six years of unremitting, heartbreaking toil. It would be hard indeed to find a comparable figure in history who, in a little over a third of a century, set so many different operations in motion, who found the time to devote his attention to minute details on one hand and on the other to cover the range of an entire planet with his plans, his instructions, his guidance and his leadership. He had laid the foundations of that future society Bahá’u’lláh had fathered upon the mind of the Master, and which He in turn had gestated to a point of perfection, passing it upon His death into the safe hands of His successor.” (Ruhiyyih Khanum, The Guardian of the Bahá’í Faith, p. 230)