In His book the Kitáb-i-‘Ahd (Book of the Covenant), Bahá’u’lláh appointed his eldest son and most devout believer as the Center of the Covenant. Here it is clearly written that ‘Abdu’l-Bahá was given full authority by Bahá’u’lláh Himself to interpret and explain His Teachings.
“The Will of the divine Testator is this: It is incumbent upon the Aghṣán, the Afnán, and My Kindred to turn, one and all, their faces towards the Most Mighty Branch. Consider that which We have revealed in Our Most Holy Book: ‘When the ocean of My presence hath ebbed and the Book of My Revelation is ended, turn your faces toward Him Whom God hath purposed, Who hath branched from this Ancient Root.’ The object of this sacred verse is none other except the Most Mighty Branch [‘Abdu’l-Bahá].” (Tablets of Bahá’u’lláh, p. 221)
‘Abdu’l-Bahá, entirely devoted to His father and wholly surrendered to the Will of God, fulfilled this role in every respect. He wrote books and prayers, gave talks, and met with the friends and followers of Bahá’u’lláh as well as the public at large. In addition to His words, ‘Abdu’l-Bahá was living proof of the divine and transformative power of His Father, showing by example in every action, large and small, wisdom, care, and love of God, deepening in every heart appreciation and understanding of this new revelation.
“He is, and should for all time be regarded, first and foremost, as the Center and Pivot of Bahá’u’lláh’s peerless and all-enfolding Covenant, His most exalted handiwork, the stainless Mirror of His light, the perfect Exemplar of His teachings, the unerring Interpreter of His Word, the embodiment of every Bahá’í ideal, the incarnation of every Bahá’í virtue, the Most Mighty Branch sprung from the Ancient Root, the Limb of the Law of God, the Being “round Whom all names revolve,” the Mainspring of the Oneness of Humanity, the Ensign of the Most Great Peace, the Moon of the Central Orb of this most holy Dispensation—styles and titles that are implicit and find their truest, their highest and fairest expression in the magic name ‘Abdu’l-Bahá. “ (Shoghi Effendi, The World Order of Bahá’u’lláh, p. 133)