Abdu’l-Baha is the Successor to Baha’u’llah and the Center of His Covenant. Shoghi Effendi is the successor to Abdu’l-Baha. Whose successor is the Universal House of Justice?
THE TWIN COVENANTS
What does Shoghi Effendi mean when he refers to the “twin Covenants” in the Baha’i Faith? These are the Covenant of Baha’u’llah, and the Covenant of `Abdu’l-Baha. They are part of the same Covenant, part of the Lesser Covenant Baha’u’llah has made with His believers. He has covenanted with us that “this is the day that will not be followed by night,” that is, that He and the Bab will guide Their Successors – the Institutions of the Cause until the appearance of the next Manifestation of God, and He has taken from us a Covenant to turn to them.
Shoghi Effendi refers to the “twin Covenants” in one of his letters to America:
“Above all, the paramount duty of deepening the spiritual life of these newly fledged, these precious and highly esteemed co-workers, and of enlightening their minds regarding the essential verities enshrined in their Faith, its fundamental institutions, its history and genesis — the twin Covenants of Baha’u’llah and of `Abdu’l-Baha … must continue to constitute the most vital aspect of the great spiritual Crusade …” 
We do not often speak of the Covenant of `Abdu’l-Baha, but it is often referred to by the Guardian, and in several letters written on his behalf, he emphasizes how important it is to understand its provisions:
“… the believers need to be deepened in their knowledge and appreciation of the Covenants of both Baha’u’llah and ‘Abdu’l-Baha.” 
“The Youth must ponder deeply over the significance and implications of the Covenants of Baha’u’llah and ‘Abdu’l-Baha, for these form the hub of the Baha’i wheel, so to speak, the point of unity and strength for all the believers all over the world. Without these Covenants the Divine Protection of God over this new world Faith would not exist. Obedience to these Covenants is the stronghold of all the Baha’is, everywhere. The Youth must grasp this fundamental truth, for this will strengthen them in their service to the Cause, as nothing else can or will.” 
The Guardian emphasized the importance of strengthening the new Baha’is in the Covenant of `Abdu’l-Baha as well as the Covenant of Baha’u’llah:
“Above all, the utmost endeavour should be exerted by your Assembly to familiarize the newly enrolled believers with the fundamental and spiritual verities of the Faith, … to instil in them a deeper understanding of the Covenants of both Baha’u’llah and ‘Abdu’l-Baha … ” 
“The unity and solidarity of its constituent members must be simultaneously reinforced, its roots permanently planted in the soil of the Covenants of both Baha’u’llah and `Abdu’l-Baha … ” 
“The deepening and enrichment of the spiritual life of the individual believer, … his understanding of the fundamentals of the Covenants established by its Author and the authorised Interpreter of its teachings…” 
Shoghi Effendi stated through his secretary that firmness in the twin Covenants is the very foundation of a Baha’i’s spiritual life:
“The thing the German believers must do is to deepen themselves in the Covenants of Baha’u’llah and `Abdu’l-Baha. Mere intellectual understanding of the teachings is not enough. Deep spirituality is essential, and the foundation of true spirituality is steadfastness in the Covenant.” 
The Master promised us “mental tests” and the Guardian promised that these intellectual tests would be safely met by studying the Will and Testament of `Abdu’l-Baha:
” … the believers need to be deepened in their knowledge and appreciation of the Covenants of both Baha’u’llah and ‘Abdu’l-Baha. This is the stronghold of the Faith of every Baha’i, and that which enables him to withstand every test and the attacks of the enemies outside the Faith, and the far more dangerous, insidious, lukewarm people inside the Faith who have no real attachment to the Covenant, and consequently uphold the intellectual aspect of the teachings while at the same time undermining the spiritual foundation upon which the whole Cause of God rests. He feels you and your dear family should do all you can to teach the believers the Will and Testament to strengthen their understanding of its important provisions; for all the authority of the administrative bodies, as well as of the Guardian himself, is mainly derived from this tremendous document.” 
The Guardian stated through his secretary that what the believers must have “more than anything else in the world” is a deeper understanding of the Master’s and Baha’u’llah’s Covenants, and that without this the teaching work would not “accomplish anything”:
“The Guardian has been considerably disturbed by the inharmony that has arisen in Germany. He feels that what the German Baha’is need — and must have — more than anything else in the world is a far deeper understanding of the Covenants of both Baha’u’llah and the Master. This is the rock foundation without which no sound super-structure can be built. Neither the administration, nor the general teaching work of the Cause in Germany, will progress, or be able to accomplish anything, unless the believers are truly firm, deep, spiritually convinced Baha’is. An intellectual grasp of the teachings is purely superficial; with the first real test such believers are shaken from the bough! But once a Baha’i has the profound conviction of the authority from God, nested in the Prophet, passed on to the Master, and by Him, to the Guardians, and which flows out through the assemblies and creates order based on obedience — once a Baha’i has this, nothing can shake him. He, therefore, urges you, and the other members of the N.G.R., to devote as much time as you possibly can, to educating the believers in the Covenant.” 
Since the beginning of the Guardian’s ministry at the time of the Master’s ascension, the people who have broken the Covenant have mostly broken the Covenant of `Abdu’l-Baha. Shoghi Effendi referred to “the breakers of Baha’u’llah’s and `Abdu’l-Baha’s Covenants” on page 58 of “Messages to America,” and in a similar vein he wrote:
“It is a pity that some of the Western friends, with remarkable naivete, do not grasp the fact that there is absolutely nothing keeping those who have broken the Covenant, whether Baha’u’llah’s or the Master’s out of the Cause of God except their own inner spiritually sick condition…” 
It lends a whole new dimension to the concept of Covenant-breaking, to realize that in this Day to be declared a Covenant-breaker is to have broken the very Covenant of `Abdu’l-Baha by turning away from the Universal House of Justice, and that “firmness in the Covenant” means obedience to the twin Covenants by turning to the Universal House of Justice.
So intimate is the relationship between the Covenant of Baha’u’llah and the Covenant of the Master, that Shoghi Effendi refers to them as one Document – one Will:
“Not so with the Revelation of Bahá’u’lláh. Unlike the Dispensation of Christ, unlike the Dispensation of Muhammad, unlike all the Dispensations of the past, the apostles of Bahá’u’lláh in every land, wherever they labor and toil, have before them in clear, in unequivocal and emphatic language, all the laws, the regulations, the principles, the institutions, the guidance, they require for the prosecution and consummation of their task. Both in the administrative provisions of the Bahá’í Dispensation, and in the matter of succession, as embodied in the twin institutions of the House of Justice and of the Guardianship, the followers of Bahá’u’lláh can summon to their aid such irrefutable evidences of Divine Guidance that none can resist, that none can belittle or ignore. Therein lies the distinguishing feature of the Bahá’í Revelation. Therein lies the strength of the unity of the Faith, of the validity of a Revelation that claims not to destroy or belittle previous Revelations, but to connect, unify, and fulfill them. This is the reason why Bahá’u’lláh and ‘Abdu’l-Bahá have both revealed and even insisted upon certain details in connection with the Divine Economy which they have bequeathed to us, their followers. This is why such an emphasis has been placed in their Will and Testament upon the powers and prerogatives of the ministers of their Faith.
“For nothing short of the explicit directions of their Book, and the surprisingly emphatic language with which they have clothed the provisions of their Will, could possibly safeguard the Faith for which they have both so gloriously labored all their lives.” 
The Greatest Holy Leaf spoke eloquently of turning to the Master’s Covenant:
“Although the towering citadel of God’s Cause is upraised on foundations of iron, and His Word is founded on authority and power, and the loyal and firm in His Covenant, through the blessings of the Abha Paradise, stand immovable as the mountains, and are fast-rooted in their love – still, the hurricanes of tests are mighty as well, and from every side comes the thundering roar of violent commotions and bitter trials. From these, at every moment each one of us should beg of God to defend and protect us.
“Let us call to mind the clear statements and the warnings revealed by the Blessed Beauty, and the explanations and commentaries of ‘Abdu’l-Baha, particularly as found in His Will and Testament. This Testament was the last song of that Dove of the Rose-garden of Eternity, and He sang it on the branch of the Tree of bestowal and grace. It was His principal gift, indeed the greatest of all splendours that radiated forth from that Day-Star of bounty, out of the firmament of His bestowals. This Testament was the strong barricade built by the blessed hands of that wronged, that peerless One, to protect the garden of God’s Faith. It was the mighty stronghold circling the edifice of the Law of God. This was an overflowing treasure which the Beloved freely gave, a goodly and precious legacy, left by Him to the people of Baha. In all the world, no gift could equal this; no dazzling gem could rival such a precious pearl.
“With His own pen, He designated as Guardian of the Cause of God, Shoghi Effendi Rabbani, the Chosen Branch, and made him the ‘blest and sacred bough that hath branched out from the Twin Holy Trees,’ to be the one to whom all must turn, the centre and focus of all on earth.
“In unmistakable terms did He set forth the obligations and elucidated the nature of the institutions of God’s Holy Faith. He laid hold of discord’s tree and brought it down. He for ever shut the door on conflicting interpretations and views. With every breath ought we to offer praise and thanks to the God of Grace for this bestowal. It is incumbent upon us to read and meditate on the contents of the Will and Testament at all times, and implore God at His Holy Threshold that He will aid us to carry out whatsoever it ordains.” 
THE TWIN SUCCESSORS
Shoghi Effendi pointed out that he is not the only successor under the Master’s Covenant; the House of Justice is co-successor with the institution of the Guardianship:
“They [Baha’u’llah and Abdu’l-Baha] have also, in unequivocal and emphatic language, appointed those twin institutions of the House of Justice and of the Guardianship as their chosen Successors, destined to apply the principles, promulgate the laws, protect the institutions, adapt loyally and intelligently the Faith to the requirements of progressive society, and consummate the incorruptible inheritance which the Founders of the Faith have bequeathed to the world.” 
 Shoghi Effendi, “This Hour, Crowded With Destiny,” letter dated August 18, 1949, “Citadel of Faith,” p. 76.
 From a letter of the Guardian’s secretary on his behalf to an individual Baha’i in Germany, The Light of Divine Guidance, Volume 2, page 84.
 From a letter of Shoghi Effendi to the National Youth Committee of the Baha’is of Germany and Austria dated 20 October 1949, “The Light of Divine Guidance,” Volume 2, page 89.
 Shoghi Effendi, Messages to Canada, pp. 61-62.
 Shoghi Effendi, Letters to Australia and New Zealand 1923-1957, p. 80.
 Shoghi Effendi, Letters to Australia and New Zealand 1923-1957, p. 93.
 From a letter of the Guardian’s secretary on his behalf to an individual Baha’i in Germany, “The Light of Divine Guidance,” Volume 2, page 83.
 From a letter of the Guardian’s secretary on his behalf to Mrs. Anna Grossman, “The Light of Divine Guidance,” Volume 2, page 84.
 From a letter of the Guardian’s secretary on his behalf to Hermann Grossman, “The Light of Divine Guidance,” Volume 2, pp. 83-84.
 From a letter on behalf of the Guardian to the National Spiritual Assembly of the United States dated April 11, 1949, Lights of Guidance, Second Edition, Page 188, #618.
 (Shoghi Effendi, The World Order of Baha’u’llah, p. 21)
 From a letter of the Greatest Holy Leaf, “Bahiyyih Khanum,” pp. 209-211.
 Shoghi Effendi, The World Order of Baha’u’llah, p. 19.