O my loving friends! After the passing away of this wronged one, it is incumbent upon the Aghsán (Branches), the Afnán (Twigs) of the Sacred Lote-Tree, the Hands (pillars) of the Cause of God and the loved ones of the Abhá 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 Aghsán, the Afnán, the Hands of the Cause of God and His loved ones must turn. He is the expounder of the words of God and after him will succeed the first-born of his lineal descendents.”
‘Abdu’l-Bahá, The Will and Testament of ‘Abdu’l-Bahá, paragraph 16

The sacred and youthful branch, the guardian of the Cause of God as well as the Universal House of Justice, to be universally elected and established, are both under the care and protection of the Abhá Beauty, under the shelter and unerring guidance of His Holiness, the Exalted One (may my life be offered up for them both). Whatsoever they decide is of God. Whoso obeyeth him not, neither obeyeth them, hath not obeyed God; whoso rebelleth against him and against them hath rebelled against God; whoso opposeth him hath opposed God; whoso contendeth with them hath contended with God; whoso disputeth with him hath disputed with God; whoso denieth him hath denied God; whoso disbelieveth in him hath disbelieved in God; whoso deviateth, separateth himself and turneth aside from him hath in truth deviated, separated himself and turned aside from God. The mighty stronghold shall remain impregnable and safe through obedience to him who is the Guardian of the Cause of God. It is incumbent upon the members of the House of Justice, upon all the Aghsán, the Afnán, the Hands of the Cause of God to show their obedience, submissiveness and subordination unto the Guardian of the Cause of God, to turn unto him and be lowly before him…”
‘Abdu’l-Bahá, The Will and Testament of ‘Abdu’l-Bahá, paragraph 17

Salutation and praise, blessing and glory rest upon that primal branch of the Divine and Sacred Lote-Tree, grown out, blest, tender, verdant and flourishing from the Twin Holy Trees; the most wondrous, unique and priceless pearl that doth gleam from out the Twin surging seas; upon the offshoots of the Tree of Holiness, the twigs of the Celestial Tree, they that in the Day of the Great Dividing have stood fast and firm in the Covenant; upon the Hands (pillars) of the Cause of God that have diffused widely the Divine Fragrances, declared His Proofs, proclaimed His Faith, published abroad His Law, detached themselves from all things but Him, stood for righteousness in this world, and kindled the Fire of the Love of God in the very hearts and souls of His servants; upon them that have believed, rested assured, stood steadfast in His Covenant and followed the Light that after my passing shineth from the Dayspring of Divine Guidance — for behold! he is the blest and sacred bough that hath branched out from the Twin Holy Trees. Well is it with him that seeketh the shelter of his shade that shadoweth all mankind.
‘Abdu’l-Bahá, The Will and Testament of ‘Abdu’l-Bahá, paragraph 2

O ye the faithful loved ones of ‘Abdu’l-Baha! It is incumbent upon you to take the greatest care of Shoghi Effendi, the twig that hath branched from and the fruit given forth by the two hallowed and Divine Lote-Trees, that no dust of despondency and sorrow may stain his radiant nature, that day by day he may wax greater in happiness, in joy and spirituality, and may grow to become even as a fruitful tree.

For he is, after ‘Abdu’l-Baha, the Guardian of the Cause of God, the Afnan, the Hands (pillars) of the Cause and the beloved of the Lord must obey him and turn unto him. He that obeyeth him not, hath not obeyed God; he that turneth away from him, hath turned away from God and he that denieth him, hath denied the True One. Beware lest anyone falsely interpret these words, and like unto them that have broken the Covenant after the Day of Ascension (of Baha’u’llah) advance a pretext, raise the standard of revolt, wax stubborn and open wide the door of false interpretation. To none is given the right to put forth his own opinion or express his particular conviction. All must seek guidance and turn unto the Center of the Cause and the House of Justice. And he that turneth unto whatsoever else is indeed in grievous error.
‘Abdu’l-Bahá, Will and Testament of `Abdu’l-Bahá, paragraph 54-55

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.
Universal House of Justice, The Kitab-i-Aqdas, p.2

Resting place of Shoghi Effendi, Guardian of the Bahá’í Faith, in the Great Northern Cemetary, London

Let us go back for a moment and recall what we were and what Shoghi Effendi was when he first assumed his function as Guardian. Those who remember the passing of the Master and the terrible blow it was to them, the intense unbearable grief it caused them, can best grasp what his feelings were. At that time he was twenty-four years of age, studying at Oxford University in England in order to better prepare himself to serve ‘Abdu’l‑Bahá as an interpreter, and to translate some of the Bahá’í literature into English, when news of the ascension reached him. Broken-hearted, so weak from suffering he had to be practically lifted from the train, he returned to Haifa. Then the second blow, as unexpected and in many ways more cruel than the first, fell upon him. The Will and Testament of His Grandfather was read to him and for the first time in his life he became apprised of the Master’s great and well-guarded secret: that he, Shoghi Effendi, the beloved, eldest grandson, was His successor and First Guardian of the Cause of God. Saddled with this great weight, crushed by this great blow, he turned his eyes to the Bahá’í world. He beheld a widely diversified, loosely organized community, scattered in various parts of the globe, and with members in about twenty countries. These people, loyal, devoted and sincere though they were, were still, to a great extent living in their parent religion’s house, so to speak; there were Christian Bahá’ís, Jewish Bahá’ís, Muhammadan Bahá’ís and so on. They believed in the Bahá’í Faith but were intimately connected with their former churches. Like fruit on a tree, they were a new crop but still stuck to the old branch. This was true East and West alike. That was the point to which the Faith had evolved at the time of the Master’s ascension.

We, on our part, beheld a young man of only twenty-four standing at the helm of the Cause and some of the friends felt impelled to advise him about what it would be wise for him to do next. It was then that we began to know not only the nature of our first Guardian but the nature of the entire Institution of Guardianship, for we quickly discovered that Shoghi Effendi was “unreachable.” Neither relatives, old Bahá’ís or new Bahá’ís, well-wishers or ill-wishers could sway his judgment or influence his decisions. We quickly came to realize that he was not only divinely guided but had been endowed by God with just those characteristics needed to build up the Administrative Order, unite the believers in common endeavor, and coordinate their world-wide activities. Shoghi Effendi immediately began to display a genius for organization, for the analysis of problems, for reducing a situation to its component parts and then giving a just and wise solution. He acted vigorously, with unflinching determination, and unbounded zeal. Those who were privileged to meet him were immediately captivated by his eager, frank and cordial attitude, by his consideration, his innate modesty, his spontaneous kindness and charm. The wheels of the Cause which had momentarily stood still at the Master’s passing, began to revolve again and at a higher tempo than ever before. Our Father, so patient, so constantly forgiving, whom we had tired and perhaps worried far more than we dreamed of, was gone, and in His stead stood our “true brother,” young, determined to see we at last got down to accomplishing the tasks set us by Bahá’u’lláh and the Master, and not willing to lose any time at all.

…What the Guardian possesses to an outstanding degree (and no doubt is divinely endowed with) is the vision of the Cause. That which might seem essential to us he would see as a purely secondary issue and what might appear in our eyes as a trivial matter might to him be the pivot of far greater decisions. He is the balance of the Cause; he weighs and classifies the problems, the requirements, the tasks of the hour, and keeps the entire Faith in every part of the world functioning as efficiently and as satisfactorily as our individual frailties and deficiencies will permit.

…Patiently, untiringly, year after year, Shoghi Effendi has labored on the rearing of the Administrative Order, using the American Bahá’í Community as the arch pattern to be followed by all other Bahá’í Communities. While the American believers struggled to learn what it really means to function as a member of an Order that has rules to be lived up to and not merely read about; while they tried to really submit their wills and conduct to the guidance of bodies conducting their affairs according to the will of the majority, the Guardian never for a moment lost sight of the purpose of his endless insistence on our following the Administrative Principles of our Faith; namely, to produce an instrument strong enough to enable us to fulfill one of the primary obligations of every believer — to teach the cause of God.

…the Cause, far from sustaining a paralyzing blow through the death of ‘Abdu’l‑Bahá, went ahead in channels prepared for its development by Him in His Will, and prospered and grew as it never had before. How immature and naive was our conception of the World Order at the time of the Master’s passing! With what paternal complacency some of the old and devoted Bahá’ís informed the Guardian that his next and wisest step would be to establish the International House of Justice! On what? He knew so well the long and thorough measures that must first be taken: Here was this great edifice, this world order, reared in the laws of Bahá’u’lláh, and its dome, the apogee, the crown, the uniting element, was the International House of Justice. Could it be hung on air? First must come the foundation, and that must be composed of not only the smallest units, the local Assemblies, but these units themselves must be made of educated material; the little cells, the individual Bahá’ís had first to understand their task and build themselves into a strong cohesive mass, capable of forming, as local Assemblies, the broad national foundation upon which the mighty pillars in turn could be reared, becoming the bones of the structure holding up the dome.
Ruhiyyih Khanum, Twenty Five Years of the Guardianship