He [‘Abdu’l-Baha] alone had been accorded the privilege of being called "the Master," an honor from which His Father had strictly excluded all His other sons. Upon Him that loving and unerring Father had chosen to confer the unique title of "Sirru'llah" (the Mystery of God), a designation so appropriate to One Who, though essentially human and holding a station radically and fundamentally different from that occupied by Bahá'u'lláh and His Forerunner, could still claim to be the perfect Exemplar of His Faith, to be endowed with super-human knowledge, and to be regarded as the stainless mirror reflecting His light.
-Shoghi Effendi (God
Passes By)