The first Guardian of Baha'i Faith Shogi Effendi wrote in his Spiritual Testament "The Dispensation of Baha'u'llah" about the continuity of guardianship in Baha'i Faith as under:
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the necessary guidance to define the sphere of the legislative action of its elected representatives would be totally withdrawn.
Severed from the no less essential institution of the Universal House of Justice this same System of the Will of ‘Abdu’l-Bahá would be paralyzed in its action and would be powerless to fill in those gaps which the Author of the Kitáb-i-Aqdas has deliberately left in the body of His legislative and administrative ordinances.
“He is the Interpreter of the Word of God,” ‘Abdu’l-Bahá, referring to the functions of the Guardian of the Faith, asserts, using in His Will the very term which He Himself had chosen when refuting the argument of the Covenant-breakers who had challenged His right to interpret the utterances of Bahá’u’lláh. “After him,” He adds, “will succeed the first-born of his lineal descendants.” “The mighty stronghold,” He further explains, “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.”
(Shoghi Effendi, The Dispensation of Baha'u'llah)