"In this Dispensation, divine guidance flows on to us in this world after the Prophet’s ascension, through first the Master, and then the Guardians."

While the Guardianship was to be a perpetual institution of the Administrative Order, it ceased to exist after the death of Shoghi Effendi because he died having violated Bahá'u'lláh's command in the Kitáb-i-Aqdas that "Unto everyone hath been enjoined the writing of a will." Having no children of his own and having declared every living male descendant of Bahá'u'lláh a Covenant-breaker, Shoghi Effendi left no eligible candidates for the office of Guardian, posing a serious problem given his assertion that "In this Dispensation, divine guidance flows on to us in this world after the Prophet’s ascension, through first the Master, and then the Guardians." He had furthermore stated in The World Order of Bahá’u’lláh that...

Divorced from the institution of the Guardianship the World Order of Bahá’u’lláh would be mutilated and permanently deprived of that hereditary principle which, as ‘Abdu’l-Bahá has written, has been invariably upheld by the Law of God. “In all the Divine Dispensations,” He states, in a Tablet addressed to a follower of the Faith in Persia, “the eldest son hath been given extraordinary distinctions. Even the station of prophethood hath been his birthright.” Without such an institution the integrity of the Faith would be imperiled, and the stability of the entire fabric would be gravely endangered. Its prestige would suffer, the means required to enable it to take a long, an uninterrupted view over a series of generations would be completely lacking, and the necessary guidance to define the sphere of the legislative action of its elected representatives would be totally withdrawn.

So fundamental was the office of the Guardianship, that Bahá’í literature was significantly altered subsequent to Shoghi Effendi's death, with the notable removal of references to "The First Guardian of the Bahá'í Faith," the "first and present Guardian," and "the lineage of succeeding Guardians." In some cases, references to the Guardian have been replaced or amended with "the Universal House of Justice" and in other instances references to the duties of the Guardian that were in the present tense have been changed to the past tense, indicating that the Guardianship has ceased.

From 'Abdu'l-Bahá's Will and Testament...

O ye beloved of the Lord! It is incumbent upon the Guardian of the Cause of God to appoint in his own life-time him that shall become his successor, that differences may not arise after his passing. He that is appointed must manifest in himself detachment from all worldly things, must be the essence of purity, must show in himself the fear of God, knowledge, wisdom and learning. Thus, should the first-born of the Guardian of the Cause of God not manifest in himself the truth of the words:—“The child is the secret essence of its sire,” that is, should he not inherit of the spiritual within him (the Guardian of the Cause of God) and his glorious lineage not be matched with a goodly character, then must he, (the Guardian of the Cause of God) choose another branch to succeed him." 

https://www.reddit.com/r/exbahai/comments/129fb9f/biggest_theological_problems_in_bahai_teaching/