Where did Shoghi Effendi derive his authority as Guardian of the Bahá’í Faith?

“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-Bahá! 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-Bahá, the Guardian of the Cause of God, the Afnán, 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.”

Bahá’u’lláh, The Kitáb-i-Adqas, p. 3: Introduction by the Universal House of Justice.