He ['Abdu'l-Baha] 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.

He is, above and beyond these appellations, the “Mystery of God”—an expression by which Bahá’u’lláh Himself has chosen to designate Him, and which, while it does not by any means justify us to assign to Him the station of Prophethood, indicates how in the person of ‘Abdu’l Bahá the incompatible characteristics of a human nature and superhuman knowledge and perfection have been blended and are completely harmonized. 

- Shoghi Effendi  (‘The Dispensation of Baha’u’llah; included in ‘The World Order of Baha’u’llah’)